Should it use it ?
It occured to me to ask when reading Bruce's release notes, which say:
| [MERGE] is similar to INSERT ... ON CONFLICT but more batch-oriented.
Currently, INSERT *never* uses bistate - even INSERT SELECT.
INSERTing 10k tuples will dirty 10k buffers - not limited to the size of a
strategy/ring buffer. Currently, MERGE will do the same.
I had a patch for INSERT last year.
https://commitfest.postgresql.org/35/2553/