Am I correct in noticing that all default Site Assets libraries can still be set on 'No versioning' like it used to be for normal libraries?
Is there a way with powershell or similar to turn to 'no versioning' for selected libraries?
Yes, it is indeed possible.
You can use
Set-PnPList to change it to
Connect-PnPOnline -Url "https://tenant.sharepoint.com/sites/testSiteCollection" Set-PnPList -Identity "Site Assets" -EnableVersioning:$false
My "Site Assets" libraries, both on-prem and O365 have major versions enabled by default.
Create major versions
There are a couple of properties that you can tweak to configure versioning:
If you're looking for disabling versioning, the line you are looking for is:
$list.EnableVersioning = $false