Landed here looking for Supabase upsert. In case this helps someone else:
I upserted using the Postgres node per this post: Implementing UPSERT/MERGE with Supabase
Just need to ensure that the column(s) you are matching are set to UNIQUE in Supabase/Postgres otherwise you wont be able to select them in the Postgres node per this post: Postgres Node Insert or Update Unique Column Name