The key thing is Postgres does handle enomem well and does a nice rollback rather than crashing the server and entering crash recovery. It’s one of the few programs that does. Exceptions for the exception.

Even a revised heuristic that only spots large, individual allocations is not going to do the job.

Oom score adjust also doesn’t do the job: because the only interesting workload is Postgres, if a backend does a page fault that needs memory, who dies? Another sibling Postgres, almost certainly. Then postmaster does crash recovery, which most would rather avoid. High performance databases with distant checkpoints can take a while to come back up.