Also, Twitter, bafflingly, allows reuse of usernames. After I deleted my (very old; original id was <1 million) Twitter account, I think it was briefly grabbed

Also, Twitter, bafflingly, allows reuse of usernames. After I deleted my (very old; original id was <1 million) Twitter account, I think it was briefly grabbed by a spammer; the username now shows as 'suspended'. So yeah, a dead placeholder is probably the safest choice.