This morning I was overcome by an urge to create a database with a specific locale, and my insufficiently caffeinated brain reminded me there was a handy LOCALE option added not so long ago, but was confused by its stubborn absence from the list of tab completion options presented by psql on a current HEAD build, no matter how much I mashed the tab key.
Further investigation confirmed the LOCALE option was added in PostgreSQL 13 (commit 06140c20) but neither commit [1] nor discussion [2] mention psql. Trivialest of trivial patches attached (will add to next CF).