I don't know when or why it stopped being a thing
The why is alluded to in Re: What's up with AnnoCPAN?. The archive suggests it stopped sometime in late 2018 or early 2019. Given the domain renewal is in March then I'd go with March 2019 as effective end date.
couldn't you roll your own with AnnoCPAN, or adding the functionality for logged in users of metacpan?
Probably, given enough time and enthusiasm. The latter isn't such an issue.
I tried previously to help out with some aspect of metacpan but the barrier to entry proved insurmountable. If time ever did permit it would be a case of rolling my own. It'll go on the to-do list next to the roll-my-own cpanratings. :-)