OSHPark’s 6 layer service here: https://docs.oshpark.com/services/six-layer/

OSHPark has a 5mil trace/ 5mil-space specification, with 4-mil annular ring and 8-mil minimum drill size (ie: 16-mil minimum via sizes). For $15/sq-inch PCB boards with no setup fees or other costs, this is easily the best US hobbyist option for BGA service.

A 16-mil via is 0.4064mm. For a 0.8mm pitch BGA, that leaves .3936mm left over on the lower layers. 5-mil space (left-side) + 5-mil trace + 5-mil space (right-side) takes up .3810mm. On the top layer, BGA contact pads vary but 0.4mm contact pads seem common.

So that’s a smooth 0.012mm left over (12 microns) for 0.80mm pitch devices. Not much room for anything smaller pitch. Still, it goes to show that all 0.8mm pitch BGAs (and larger pitched devices) are now easily available to hobbyists.