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In Postgres 12, the pluggable storage API was introduced. That allows use of multiple different storage engines with Zheap being one of the new storage engines.

IIRC, one of the big reasons for implementing pluggable storage was for Zheap.

FWIW, the existing implementation has worked really well for most use cases.

It's a complex tradeoff. It depends on the specific workload which scheme is better. There's no clear winner. This is why it's being introduced as a pluggable choice vs a wholesale change.

Andy Pavlo's class has a lot of good general information on the topic: https://15721.courses.cs.cmu.edu/spring2020/slides/03-mvcc1....

To put it simply (very simply), most of the time you don't need old rows (rollbacks are rare, crashes are rare, etc.). If you store old rows (undo) in a separate place, you don't have to read (and skip) them all the time while reading actual data. That's the main benefit.