I'm trying to figure out if the following stored procedure is safe in a high concurrency environment.
PgAdmin-Support is for issues specific to PgAdmin.
Further, Concurrency is a relatively complicated topic, but more importantly I am not sure if PgAdmin-Support is the right place to request help for this query.
You should probably be looking at an appropriate PostgreSQL mailing lists for this query, although without putting in much thought / effort / input from your end, and without asking a query that isn't already clear in the docs, you'd be lucky to get the answer you're looking for.