No

Is there such a mod for SC1?

According to their readme, it works with the newest Jelllyfin server.

https://gitlab.futo.org/keyboard/latinime

Debatable how “proprietary” it really is.

You can probably use an Elgato Facecam (Mk.2), configure it in a Windows VM and then save the settings directly on the device to use it under Linux.

Does this support other targets like twitch VODs?

Huh, you’re right. But a few weeks ago I was definitely able to download via Fido. Maybe try that.

Nah, that was public opinion on /r/datahoarders (I think) back when I still used reddit.

People complain that they won’t respect their lifetime plans forever, but otherwise I haven’t heard anything bad about them.

Never had an issue getting an iso from them via Mullvad.

Mattermost does not have E2EE to my knowledge.

Not E2EE though, or at least not fully.

XMPP only does it with certain client extensions. And Matrix only does it when the rooms are set up this way. SimpleX does what you want, but is kind of unintuitive for the average user.
I say go with Signal, it does what you want and is idiot-proof.

Third time’s the charm! /s

But as a third party you can not know which clients are using this outdated http mirror. On BitTorrent you can see every participating peers and some of them are probably enduser machines (depending on the actual implementation of OP’s suggestion).

https://posteo.de or https://mailbox.org

using your simplex profile on both desktop and phone

Why would that be hard?

Wow, I think this basically makes Session obsolete.

How do I join the closed beta?

I didn’t have much luck with Bottles in the past, but this is entirely anecdotal.