Is there a maximum number of rows that could be deleted here?
Would it make sense to add a limit on the number of rows deleted?
My thinking is that we could choose a reasonably large upper limit and hopefully converge after a few iterations.

Not a bad idea. Limiting would only be relevant for the first time(s) this query runs, after that there's no point but if we want to avoid the initial performance penalty of doing a huge delete, that would be sensible. There's no limit for deletes though so we could rely on selecting the min(created_at) and only delete 1 (or N days) of data.

I thinking deleting the oldest day of data every interval will probably work fine here. The magnitude of that data should scale with the size of the deployment. We'll have to assume that the database is correctly sized to handle that level of deletes, but if not that is likely a sizing issue with the database itself in relation to the rest of the deployment.

This change may be little too soon to drop. If customers identify problems with the release, they will decide to roll back to the previous version. Boom! Noworkspace_agent_stats anymore.

I agree, I made this a separate PR so we can defer it for later.

Rollbacks require db backups at present. We just do not support downgrades at present.

And not supported like, you can't even really do it. You have to run themigration downgrade with the latest version, then rollback. We have 0 documentation for this, so no customer should do it.