Disabling it doesn't really work right. It doesn't let you still see history from when you joined; it just lets you see whatever bits of history your client has cached, which resets anytime you restart the client (on desktop) or anytime the client loses state due to memory pressure (on Android, no idea about iOS.) It doesn't really make much sense.

I guess the desktop behavior kind of matches IRC, but it's not what Discord users expect; the mobile behavior is just random, since you can't control when the client is "connected" or not.

That's a good point and now that you mention it I remember seeing that option. But as you say, it's almost always on, so it barely matters.

To be clear, I'm not saying Discord or any server admin are making a mistake here, scrollback is often very useful. But I've definitely opted not to share some things because I knew they would be searchable ultimately. Having witnessed the transition of some IRC channels to Discord, I dare say others must have felt something similar.

im in a few servers with it off and it works really poorly. even switching between channels in same session, especially on web will instantly clear it, they seem to struggle with session management