LGTM. Naively I'd expect this change to include a db schema change that drops this column from db-next -- is there a reason we're not doing that yet?

The path forward for removing the column didn’t feel solid enough to justify a schema change at this time. However, if you support modifying the table now, I’m happy to create another PR with the following migration:

-- +migrate Up
-- SQL in section 'Up' is executed when this migration is applied

ALTER TABLE `registrations`
DROP COLUMN `initialIP`,
DROP KEY `initialIP_createdAt`;

-- +migrate Down
-- SQL section 'Down' is executed when this migration is rolled back

ALTER TABLE `registrations`
ADD COLUMN `initialIP` binary(16) NOT NULL DEFAULT '\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0',
ADD KEY `initialIP_createdAt` (`initialIP`, `createdAt`);

Yeah, I think doing that schema change now would be good, as "proof" that it's safe for SRE to drop the column (however they choose to do so) in Staging and then in Prod.