As per the spec each zone must have a
SOA record, even though the usage is somewhat situational.
I suppose what you read a bit too much into in the answers to the other question is that, while it's not optional, if you were to violate the spec in this regard (I see no reason why you would want to) it will only break some things, not everything.
From what I can see at this time, the statement
0-21.ru doesn't have an SOA record is not obviously true.
The actual situation at this time seems to be that all the nameservers for
0-21.ru are unresponsive, making it impossible to look up any records at all. Ie, it's not possible to tell if there is a
SOA record or not.
As for the other aspect of your question, you could check for the existence of a delegation in the parent zone.
dig +trace +all 0-21.ru NS would show the chain of delegations. Normally you would see the responses all the way to the authoritative nameservers, but in this case it currently ends at delegation from the
ru nameservers as the authoritative servers for
0-21.ru are MIA.
Or, for that matter, if you know the nameservers of the parent zone (in this case
ru) you could do eg:
$ dig @d.dns.ripn.net 0-21.ru +norec
; <<>> DiG 9.11.20-RedHat-9.11.20-1.fc32 <<>> @d.dns.ripn.net 0-21.ru +norec
; (2 servers found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 52492
;; flags: qr; QUERY: 1, ANSWER: 0, AUTHORITY: 2, ADDITIONAL: 3
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;0-21.ru. IN A
;; AUTHORITY SECTION:
0-21.ru. 345600 IN NS ns1.0-21.ru.
0-21.ru. 345600 IN NS ns2.0-21.ru.
;; ADDITIONAL SECTION:
ns1.0-21.ru. 345600 IN A 18.104.22.168
ns2.0-21.ru. 345600 IN A 22.214.171.124
;; Query time: 80 msec
;; SERVER: 2001:678:18:0:194:190:124:17#53(2001:678:18:0:194:190:124:17)
;; WHEN: Fri Jul 03 10:54:24 UTC 2020
;; MSG SIZE rcvd: 104
To see that there does indeed exist a delegation.