Is it possible to use RLS without using the supabase client?

RLS itself is a Postgres feature, but the set of auth functions are helpers brought in as part of gotrue. You can write similar functions to access any header passed into PostgREST ( see their docs) which is Supabase API for the database. https://postgrest.org/en/stable/auth.html#overview-of-role-system https://postgrest.org/en/stable/api.html#http-context