If you have trouble using some XMPP client you should join their XMPP chat group and ask for assistance.

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I don’t think so, but XMPP servers in general don’t block tor exit nodes. There are some XMPP servers that have .onion addresses, checkout this list for example, I would recommend to always enable OMEMO end to end encryption and delete your account once you are done with it, you can always create a new account.

Matrix is a metadata centralization network, I mean although it is decentralized, the level of metadata that is shared between servers is not good. Also checkout Matrix? No, thanks.

Better use good old XMPP there are many public servers. Realistically though, any server host would probably do exactly what Protonmail did here. This organization’s XMPP service seems like a good option though.

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Disroot.org is not an activist organization, they will do the same thing.