The exact documentation is here, and the statement is a bit vague regarding software installation; it simply says something like “it should be fine.”
And then I find threads where the complete opposite is done.
I’m not objectively sure if using apt install is okay, but does that mean I can also add third-party repositories to templates to install programs that use independent maintainers? Like importing the key and everything, as in Ubuntu? And wouldn’t that break the template?