Applause, which is in critique for destroying good software (like with komoot).

AfaIk, Komoot has been bought by Bending Spoons, not Applause.

Surely depends on your usecase. I’ve replaced it with Xournal++.

As others have mentioned, it’s a way to link to a community. More specifically, it’s what allow to share a link to a community (not to a specific post, just the community itself) that will work no matter what instance anyone is clicking it from, unlike your usual link.

Specifically, !buyfromeu@feddit.org links to the local copy on your instance of /c/buyfromeu@feddit.org and thus, allows you to post and comment using your account on your instance, whereas https://feddit.org/c/buyfromeu@feddit.org links directly to feddit.org and would only allow you to post and comment if you have an account on feddit.org.