I've updated my earlier comment to clarify that I was checking the changelog for apt in my Debian system. On review it doesn't make much sense - searching for

I've updated my earlier comment to clarify that I was checking the changelog for apt in my Debian system. On review it doesn't make much sense - searching for 'phase' there's an 'Add support for phased updates' in 2.5.3, but a few months earlier, in 2.5.1, the comment is: "(Temporarily) Rewrite phased updates using a keep-back approach." I thought I'd done a case-insensitive search before, but obviously hadn't, as there's one earlier reference in 2021-01 (for v2.1.16), which kind of aligns with the askubuntu's historical mention of 2.1.16 on the timeline, but the description certainly doesn't sound like the base feature (phased updates) introduction. "Add support for Phased-Update-Percentage, previously used only by update-manager."