It's been reserved by ICANN: https://www.icann.org/en/board-activities-and-meetings/materials/approved-resolutions-special-meeting-of-the-icann-board-29-07-202

It's been reserved by ICANN: https://www.icann.org/en/board-activities-and-meetings/materials/approved-resolutions-special-meeting-of-the-icann-board-29-07-2024-en#section2.a https://www.icann.org/en/board-activities-and-meetings/mater... > Resolved (2024.07.29.06), the Board reserves .INTERNAL from delegation in the DNS root zone permanently to provide for its use in private-use applications.