It’s in beta, but it’s already quite usable. You can try it without account on a public instance e.g. at call.element.io

mu4e+mbsync+msmtp

That could be very useful when using a text-based web browser (e.g. TUI)

I am not an expert, but I see that the Apache license has no copyleft.

Have you heard of Beeper?

I think Matrix has bridges to all these networks, except ActivityPub (it may, I just don’t know).

I don’t know about OP, but it is available in https://thecapslock.gitlab.io/fdroid-patched-apps/fdroid/repo and https://calyxos.gitlab.io/calyx-fdroid-repo/fdroid/repo

I have seen many different ways to achieve that in Emacs. Do you have one to recommend (modes, packages)? Do you synchronise calendars with other devices (e.g. smartphone, server)?

There is a payment option inside the app. It uses Stripe to process the payment.

Matrix is in the process of being adopted as the MIMI standard by the IETF. At least it was one or two months ago.

There is also Element Call at call.element.io. I have not tried screen-sharing but it’s worth a look.

obligatory xkcd

FairEmail is the best email app I’ve ever used.

qpdf is the best CLI tool that I have used to edit the structure of a PDF. I recommend it if you don’t require a GUI, and do not need to edit the content.