Can I delete iMessages conversations across all devices.... yet? ever?

Great. Thank you. Hopefully that works.

I had iCloud for iMessages disabled on my phone. Was already enabled on my Mac. I clicked "sync" after enabling everywhere and nothing happened. I also explicitly ran iCloud backup from my phone and when that was complete clicked "sync" again on the Mac, but still nothing nothing happened. Perhaps it will work later as iCloud sync is not the swiftest.

I am mystified why enabling iCloud for iMessages is apparently totally different than signing into iMessages for the same iCloud account.

Edit: Figured out that this works with messages going forward, which is great. There doesn't seem to be a way to sync old messages. The old messages must be stored locally so I tried removing user/library/Messages and then restarting Messages but they all came right back. My MacBook is a 2013 so I guess eventually I'll get a new one so that will fix it :/

Anyway, thank you again for your help. I am still frustrated with Apple, however. I've had this problem going back as far as Messages has existed on Mac (a decade ago?). So apparently sometime in the last half decade or so it was fixed but there was never any indication that a fix was made or that users had to change any settings to enable it. A simple dialog with "Your messages are not syncing. To enable syncing, do this.... [ ] Do not show again" would have been wonderful.