Hello PW, Thank you for posting in our Q&A forum.
Are GPO Group Policies somehow applicable for local users on one computer without active directory domain?
A: If it is a user configuration, and you configured this user setting via local group policy, each user that login to this machine will apply this user setting.
If it is a computer configuration, and you configured this computer setting via local group policy, no matter any user that login to this machine, this computer setting will apply to this machine.
for example: I don't even know how to disable standby modes S1-S3 for a local user.
A: Did you mean the policy "Allow standby states(S1-S3) when sleeping (Plugged in/ on battery)", if so, you can check this thread below you posted.
Hope the information above is helpful.
If you have any question or concern, please feel free to let us know.
Best Regards,
Daisy Zhou