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Slack still has the same realtime and RPC-style APIs.

It's also actively fighting against just writing an alternative client, IIRC from my experiences trying to run weechat-slack

My experience with wee-slack is pretty good, actually. Although lately I've been running the "native" app.

Are they usable for the purpose of writing a client?

No, they seem explicitly designed to avoid interactive use an end user.

Ah, that's what I felt. Thank you.