A proper use of psubst would require proper UNC path notation:
psubst D: \\PC1\D$\soft /P
The psubst utility (https://github.com/ildar-shaimordanov/psubst) operates like the old DOS subst command to map a network share to a local drive letter. Persistence (to cause the drive to reappear after a reboot) is supposed to be applied with the /P command line option. If it didn't work for you, maybe you were trying to use it on an operating system where that's not supported. Better alternatives for current Windows operating systems today are the "net use" command, and the registry.
For a working demonstration of a mapped drive for modern Windows operating systems using the "net use" command, see How do I map a drive letter to a network share for all RDP users on a Windows Server 2016. The instructions also apply to Windows 10. Basically you'd create a text file drive-d.bat and place it in C:\ProgramData\Microsoft\Windows\Start Menu\Programs\StartUp, and then reboot:
net use D: /delete
net use D: \\PC1\D$\soft
You may also want to consider a registry hack instead. To do so, first create a text file called drive-d.reg. You can save it anywhere. Then copy and paste into it the following content:
REGEDIT4
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\DOS Devices]
"D:"="\\\\PC1\\D$\\soft"
Once you've saved the file and you're viewing it while sitting inside Windows Explorer, double-click it as if to launch it. Accept the prompts that appear and reboot your machine afterwards.
See also: