> Signal is an open protocol and supports E2E encrypted text, voice and video.

It's open in name but proprietary. How many competing open source clients exist? Right.

Also, it's extremely limited like every other proprietary system, by the fact that the client is proprietary and thus can only do what they feel like supporting.

How do I pipe incoming signal messages to a script which will process it in ways I like? How do I programmatically send messages based on the output of other scripts? And so on.

All this is trivially easy functionality with email because it is an open protocol.

There were 2 important adjectives before open.

What about secure and encrypted, which I feel are even more important adjectives?

I think they're important too. But it isn't responsive to what they said.

wasn't signal shutting down alternative clients or something?

Just the ones connecting to their servers.

The servers the vast majority of other users connect to.