Today Postgres has
insert ... on conflict, and upsert has mostly historical Postgres questions, while new questions about "insert ... on conflict" are for the most part not tagged with any specific tag about this feature.
One view is that tags should be more generic so that users not knowing exactly what they are looking for can find things -- for example users knowing syntax/concept/feature from one DBMS looking for help about that on another, bearing in mind that searching for a specific DBMS often requires eg "Postgres" or postgres. In this scenario we could make upsert and merge synonyms, and add something like insert-on-conflict as a synonym too.
In the other view we keep tags as specific as possible, probably with links to other tags in their descriptions and wiki entries. In this case we add insert-on-conflict as a separate tag, and all questions about "inserting if new, updating if existing" on modern versions of Postgres should probably be tagged with this.