I believe Namecheap only offers registrar lock (clientUpdateProhibited), not registry (serverUpdateProhibited). There are very few registrars that offer registry lock and they're all "enterprise", probably because the relationship required for registry unlock protocols to work doesn't scale with retail customers.

Off the top of my head: NetSol, MarkMonitor, CSC, maybe Cloudflare. There are more that will do it for specific ccTLDS (.ca has lots, for example).