You mostly see registry lock offered by corporate registrars that have a high touch customer service flow, e.g. you have a dedicated person servicing your account.
Yeah, CloudFlare is one that offers it, but it seems firmly in "Request Quote/Call Sales" territory for pricing. It dates from when they first intro'd their original registrar offering [1] a few years ago, and they said right out:
>"CloudFlare Registrar is not designed for the masses. There are plenty of great mass-market registrars. However, if you’re an organization where losing your domains would be a front-page story, then CloudFlare Registrar is for you."
Of course, they later did introduce a mass market offering as well, but not with that set of features! :)
Tangent: The mass market offering from Cloudflare does not have a way to buy/register a new domain. It’s been way too long since that offering was made public and it’s still stuck with just transferring domains from another registrar into Cloudflare.
