I played a bit of 4.51 preview with Yor, and a custom Synthetic race with Cybernetic and Slaver traits. Here are my observations:
Homeworld and Colonies list pop-caps of +10 and +3, respectively, but actually get a flat 8 max population. There seems to be a new stat, Synthetic Population Cap; this could be more clear to players, that it is a separate stat from Population Cap.
Cities are working! Available from the start without any tech, and receive an adjacency bonus from population-boosting improvements. They DO list the separate Synthetic Population Cap, thank you.
Still can't build Upgraded Colony Capital, even though Advanced is on the tech tree... but I don't really care anymore, as long as there is some reasonable way to raise Yor pop-cap.
Information Processor gives +2% Research at a cost of 1 Maintenance, gives out both Research and Ship Construction adjacency bonus, but receives no Research adjacency bonus levels at all. Working as intended? What is even the point of this improvement, when Yor can't build Starports and the only other real Ship Construction building is Strategic Command, which requires Helios Ore?
Custom Races Generally:
Still can't get both a starting Survey Ship and Scouts. Have to pick one.
Synthetic/Slaver Custom Race:
Forced Labor Camp is a Social improvement, but upgrades to Slave Camp, which is All Construction.
None of the Slaver buildings increase Synthetic Population Cap, but I suspect this is intentional? The only way to raise Synthetic Population Cap is by building Cities?
Planet Class seems not to raise Population Cap anymore. I did not notice at which version this became the case.