I can't upvote or comment, due to insufficient reputation but tzafrir's answer is the closest there is to the way to actually reverse what you have just uninstalled.
I have used a shorter form, in the past and it worked fine for me. Always.
apt install $(grep Remove /var/log/apt/history.log | tr ',' '\n' | cut -d':' -f1 | tr '\n' ' ' | cut -d' ' -f3-)
Where grep Remove /var/log/apt/history.log
is the following:
/home/andrei# grep Remove /var/log/apt/history.log
Remove: python3.8-venv:amd64 (3.8.10-0ubuntu1~20.04.7), python3-pexpect:amd64 (4.6.0-1build1), python3-distro-info:amd64 (0.23ubuntu1), python3-dev:amd64 (3.8.2-0ubuntu2), python3-deprecated:amd64 (1.2.7-1), gitsome:amd64 (0.8.0+ds-4), python3-json-pointer:amd64 (2.0-0ubuntu1), python3-roman:amd64 (2.0.0-3build1), netplan.io:amd64 (0.104-0ubuntu2~20.04.2), python3-dulwich:amd64 (0.19.15-1build1), cloud-init:amd64 (23.1.1-0ubuntu0~20.04.1), python3-mimeparse:amd64 (1.6.0-3build1), landscape-common:amd64 (19.12-0ubuntu4.3), python3-unittest2:amd64 (1.1.0-6.1build1), update-manager-core:amd64 (1:20.04.10.11), networkd-dispatcher:amd64 (2.1-2~ubuntu20.04.3), python3-zope.interface:amd64 (4.7.1-1), python3-distutils:amd64 (3.8.10-0ubuntu1~20.04), gvfs-backends:amd64 (1.44.1-1ubuntu1.2), python3-gpg:amd64 (1.13.1-7ubuntu2), python3-linecache2:amd64 (1.0.0-4ubuntu1), python3-olefile:amd64 (0.46-2), python3-babel:amd64 (2.6.0+dfsg.1-1ubuntu2.2), python3-software-properties:amd64 (0.99.9.11), python3-icu:amd64 (2.4.2-0ubuntu3), pyzor:amd64 (1:1.0.0-3), python3-sigmavirus24-urltemplate:amd64 (3.0.1-1), byobu:amd64 (5.133-0ubuntu1), python3-wadllib:amd64 (1.3.3-3build1), python3-setproctitle:amd64 (1.1.10-1ubuntu1), update-notifier-common:amd64 (3.192.30.17), python3-sphinx:amd64 (1.8.5-7ubuntu3), python3-distro:amd64 (1.4.0-1), python3-jwt:amd64 (1.7.1-2ubuntu2.1), python3-configargparse:amd64 (0.13.0-2), python3-ldb:amd64 (2:2.4.4-0ubuntu0.20.04.1), python3-systemd:amd64 (234-3build2), python3-josepy:amd64 (1.2.0-2), brz:amd64 (3.0.2-4ubuntu2), bzr:amd64 (2.7.0+bzr6622+brz), pastebinit:amd64 (1.5.1-1), ubuntu-advantage-tools:amd64 (27.13.6~20.04.1), python3-urllib3:amd64 (1.25.8-2ubuntu0.2), certbot:amd64 (0.40.0-1ubuntu0.1), python3-zope.hookable:amd64 (5.0.0-1build1), language-selector-common:amd64 (0.204.2), python3-jsonpatch:amd64 (1.23-3), python3-subunit:amd64 (1.4.0-0ubuntu1), python3-filelock:amd64 (3.0.12-2), python3-pbr:amd64 (5.4.5-0ubuntu1), ubuntu-standard:amd64 (1.450.2), python3-pil:amd64 (7.0.0-4ubuntu0.7), python3-pip:amd64 (20.0.2-5ubuntu1.8), python3-ply:amd64 (3.11-3ubuntu0.1), python3-keyring:amd64 (18.0.1-2ubuntu1), python3-wheel:amd64 (0.34.2-1ubuntu0.1), python3-entrypoints:amd64 (0.3-2ubuntu1), python3-traceback2:amd64 (1.4.0-5ubuntu1), python3-netifaces:amd64 (0.10.4-1ubuntu4), python3.8:amd64 (3.8.10-0ubuntu1~20.04.7), fail2ban:amd64 (0.11.1-1), open-vm-tools:amd64 (2:11.3.0-2ubuntu0~ubuntu20.04.4), python3-gssapi:amd64 (1.6.1-1build1), python3:amd64 (3.8.2-0ubuntu2), python3-service-identity:amd64 (18.1.0-5build1), python3-idna:amd64 (2.8-1), python3-wrapt:amd64 (1.11.2-2build1), python3-rpm:amd64 (4.14.2.1+dfsg1-1build2), python3-augeas:amd64 (0.5.0-1.1), python3-zope.event:amd64 (4.4-2build1), python3-testtools:amd64 (2.4.0-0ubuntu1), python3-lazr.restfulclient:amd64 (0.14.2-2build1), plymouth-theme-ubuntu-text:amd64 (0.9.4git20200323-0ubuntu6.2), python3-six:amd64 (1.14.0-2), python3-github:amd64 (1.43.7-1), ubuntu-server:amd64 (1.450.2), python3-jsonschema:amd64 (3.2.0-0ubuntu2), python3-numpydoc:amd64 (0.7.0-2), python3-lazr.uri:amd64 (1.0.3-4build1), hibagent:amd64 (1.0.1-0ubuntu1), python3-zope.component:amd64 (4.3.0-3), python3-prompt-toolkit:amd64 (2.0.10-2), python3-breezy:amd64 (3.0.2-4ubuntu2), python3-appdirs:amd64 (1.4.3-2.1), python3-mock:amd64 (3.0.5-1build1), python3-requests:amd64 (2.22.0-2ubuntu1), python3-distlib:amd64 (0.3.0-1), python3-httplib2:amd64 (0.14.0-1ubuntu1), python3-gitlab:amd64 (1:2.0.1-1), python3-fastimport:amd64 (0.9.8-5build1), python3-pyparsing:amd64 (2.4.6-1), python3-pyinotify:amd64 (0.9.6-1.2ubuntu1), python3-pymacaroons:amd64 (0.13.0-3), python3-twisted:amd64 (18.9.0-11ubuntu0.20.04.2), python3-simplejson:amd64 (3.16.0-2ubuntu2), python3-imagesize:amd64 (1.2.0-1), python3-importlib-metadata:amd64 (1.5.0-1), samba-libs:amd64 (2:4.15.13+dfsg-0ubuntu0.20.04.1), ubuntu-minimal:amd64 (1.450.2), apport:amd64 (2.20.11-0ubuntu27.25), ec2-hibinit-agent:amd64 (1.0.0-0ubuntu9.2), python3-twisted-bin:amd64 (18.9.0-11ubuntu0.20.04.2), python3-future:amd64 (0.18.2-2ubuntu0.1), python3-cryptography:amd64 (2.8-3ubuntu0.1), python3-distupgrade:amd64 (1:20.04.41), python3-zipp:amd64 (1.0.0-1), python3-debconf:amd64 (1.5.73), python3-magic:amd64 (2:0.4.15-3), python3-commandnotfound:amd64 (20.04.6), xfsprogs:amd64 (5.3.0-1ubuntu2), python3-update-manager:amd64 (1:20.04.10.11), ubuntu-release-upgrader-core:amd64 (1:20.04.41), python3-apport:amd64 (2.20.11-0ubuntu27.25), python3-docopt:amd64 (0.6.2-2.2ubuntu1), python3-constantly:amd64 (15.1.0-1build1), python3-cffi-backend:amd64 (1.14.0-1build1), python3-alabaster:amd64 (0.7.8-1build1), python3-click:amd64 (7.0-3), python3-enchant:amd64 (2.0.0-4), python3-docutils:amd64 (0.16+dfsg-2), python3-incremental:amd64 (16.10.1-3.2), python3-venv:amd64 (3.8.2-0ubuntu2), python3-markupsafe:amd64 (1.1.0-1build2), ufw:amd64 (0.36-6ubuntu1), python3-acme:amd64 (1.1.0-1), xonsh:amd64 (0.9.13+dfsg-2), python3-configobj:amd64 (5.0.6-4), python3-more-itertools:amd64 (4.2.0-1build1), python3-blinker:amd64 (1.4+dfsg1-0.3ubuntu1), python3-decorator:amd64 (4.4.2-0ubuntu1), python3-lib2to3:amd64 (3.8.10-0ubuntu1~20.04), python3-yaml:amd64 (5.3.1-1ubuntu0.1), python3-pkg-resources:amd64 (45.2.0-1ubuntu0.1), python3-breezy.tests:amd64 (3.0.2-4ubuntu2), python3-openssl:amd64 (19.0.0-1build1), python3-kerberos:amd64 (1.1.14-3.1build1), python3-hyperlink:amd64 (19.0.0-1), python3-rfc3339:amd64 (1.1-2), python3-requests-toolbelt:amd64 (0.8.0-1.1), libsmbclient:amd64 (2:4.15.13+dfsg-0ubuntu0.20.04.1), python3-certbot-apache:amd64 (0.39.0-1), sosreport:amd64 (4.4-1ubuntu0.20.04.1), python3-dbus:amd64 (1.2.16-1build1), python3-talloc:amd64 (2.3.3-0ubuntu0.20.04.1), lsb-release:amd64 (11.1.0ubuntu2), python3-certbot:amd64 (0.40.0-1ubuntu0.1), python3-gi:amd64 (3.36.0-1), python3-paramiko:amd64 (2.6.0-2ubuntu0.1), python3-tz:amd64 (2019.3-1ubuntu0.20.04.0), python3-ptyprocess:amd64 (0.6.0-1ubuntu1), unattended-upgrades:amd64 (2.3ubuntu0.3), python3-automat:amd64 (0.8.0-1ubuntu1), python3-oauthlib:amd64 (3.1.0-1ubuntu2), python3-launchpadlib:amd64 (1.10.13-1), python3-pygments:amd64 (2.3.1+dfsg-1ubuntu2.2), command-not-found:amd64 (20.04.6), ssh-import-id:amd64 (5.10-0ubuntu1), python3-nacl:amd64 (1.3.0-5), python3-hamcrest:amd64 (1.9.0-3), python3-pyrsistent:amd64 (0.15.5-1build1), supervisor:amd64 (4.1.0-1ubuntu1), python3.8-dev:amd64 (3.8.10-0ubuntu1~20.04.7), python3-wcwidth:amd64 (0.1.8+dfsg1-3), python3-requests-unixsocket:amd64 (0.2.0-2), python3-bcrypt:amd64 (3.1.7-2ubuntu1), python3-jinja2:amd64 (2.10.1-2), rpmlint:amd64 (1.11-0.2), python3-certbot-nginx:amd64 (0.40.0-0ubuntu0.1), python3-debian:amd64 (0.1.36ubuntu1), python3-gdbm:amd64 (3.8.10-0ubuntu1~20.04), python3-fixtures:amd64 (3.0.0-3build1), python3-newt:amd64 (0.52.21-4ubuntu2), python3-certifi:amd64 (2019.11.28-1), cloud-guest-utils:amd64 (0.31-7-gd99b2d76-0ubuntu1), python3-parsedatetime:amd64 (2.4-5), python3-secretstorage:amd64 (2.3.1-2ubuntu1), python3-chardet:amd64 (3.0.4-4build1), python3-problem-report:amd64 (2.20.11-0ubuntu27.25), python3-colorama:amd64 (0.4.3-1build1), python3-virtualenv:amd64 (20.0.17-1ubuntu0.4), python3-serial:amd64 (3.4-5.1), python3-pyasn1-modules:amd64 (0.2.1-0.2build1), python3-extras:amd64 (1.0.0-4build1), python3-packaging:amd64 (20.3-1), python3-apt:amd64 (2.0.1ubuntu0.20.04.1), python3-setuptools:amd64 (45.2.0-1ubuntu0.1), python3-attr:amd64 (19.3.0-2), python3-pyasn1:amd64 (0.4.2-3build1), python3-feedparser:amd64 (5.2.1-2), dh-make:amd64 (2.201903), software-properties-common:amd64 (0.99.9.11)
Full filter result here
Which was caused by sudo apt remove python3.8