Great change!

Importantly, in the replies it is noted by @pfrazee.com that the change is not retroactive to folks already verified on a custom domain: bsky.app/profile/pfra...

as of this week, when you verify your account by updating your username from a default like emily.bsky.social to your website @emilyliu.me, your former bsky.social username is reserved for you automatically (and prevents impersonators from claiming it). you can revert back to it at anytime

How to verify your Bluesky account - Bluesky
Here's how to verify your Bluesky account by setting your website as your username.

— Emily Liu (December 17, 2024 at 4:34:55 PM UTC)