to be fair sql server's holdlock is pretty specific to sql server, there's this note on pg's merge project page (https://wiki.postgresql.org/wiki/SQL_MERGE)

> PostgreSQL doesn't have a good way to lock access to a key value that doesn't exist yet--what other databases call key range locking (SQL Server for example). Improvements to the index implementation are needed to allow this feature.

as of 2017 at least the plan was to implement merge using the underlying insert on conflict mechanism and I think everyone agrees that the only reasonable expected behavior from merge is atomic upserts (and the pk violations from concurrent merges on sql server and oracle really should be recognized as bugs!) so maybe postgres merge will be the first non-buggy implementation w/o hints. reading the related threads exposes a surprising amount of complexity though so who knows.