Should we remove vacuum_defer_cleanup_age?

Hi,

As evidenced by the bug fixed in be504a3e974, vacuum_defer_cleanup_age is not
heavily used - the bug was trivial to hit as soon as vacuum_defer_cleanup_age
is set to a non-toy value. It complicates thinking about visibility horizons
substantially, as vacuum_defer_cleanup_age can make them go backward
substantially. Obviously it's also severely undertested.

I started writing a test for vacuum_defer_cleanup_age while working on the fix
referenced above, but now I am wondering if said energy would be better spent
removing vacuum_defer_cleanup_age alltogether.

vacuum_defer_cleanup_age was added as part of hot standby. Back then we did
not yet have hot_standby_feedback. Now that that exists,
vacuum_defer_cleanup_age doesn't seem like a good idea anymore. It's
pessimisistic, i.e. always retains rows, even if none of the standbys has an
old enough snapshot.

The only benefit of vacuum_defer_cleanup_age over hot_standby_feedback is that
it provides a limit of some sort. But transactionids aren't producing dead
rows in a uniform manner, so limiting via xid isn't particularly useful. And
even if there are use cases, it seems those would be served better by
introducing a cap on how much hot_standby_feedback can hold the horizon back.

I don't think I have the cycles to push this through in the next weeks, but if
we agree removing vacuum_defer_cleanup_age is a good idea, it seems like a
good idea to mark it as deprecated in 16?

Greetings,

Andres Freund