(Sorry for the late reply.) Our invite secrets implementation on toot.cafe is a bit hacky (and it also has some small bugs), which is why I didn’t propose it to the main repo. Also what we really want is the ability for users to invite other users, and the password system was just the easiest way to do that.
For instance whitelisting, yes I would love to see that in the main repo, or at the very least have it easily-available to merge into an instance.
Maybe there can be a community solution to this? E.g. we can fork the Mastodon repo and create a README describing some of the interesting branches, with links to those branches so that admins can easily merge them in if they want. No need to have anything and everything in the main repo.
Think of it as a “Mastodon add-ons page for instance owners.”