With android 16 introducing "mid releases" (QPR2), they expect OEMs to start shipping those as well, QCOM already has a QPR2 BSP release, and Samsung is expected to release QPR2 based builds soon.
As far as contributions go, google usually wanted patches to apply to main, I don't think that ever changed. And even there now that AOSP development is fully closed, it's even easier as partners will likely just upload patches against internal main instead. Less integration work there as well.
There really isn't a good explanation as to why they want to do move code drop cadence, other than they can and want to avoid wasting time releasing QPR1/3 that no OEM ever shipped (expect Pixels that is)
In other words, the result is still open, but the development process is not.
[edit] based on the other comments, I surmise that public pushes were already infrequent.
Is the source code available at all times? This is a genuine question, I don't know right now.
When it comes to large bureaucracies, always assume laziness over malice or strategic competence.