When, precisely, did the US stop doing that?

The US Air Force has developed, tested, and is now in early production of the B-21, a long-range stealth bomber, one of their most important projects.

Like B21?

B-52 is still very much active. As for stealth, B-2.

Aren't they just waiting for the active camouflaging technology to catch up? Normally, we think about that as something on the underside of the aircraft so that it appears invisible from the ground... but the same principle could apply to the top side of the aircraft if it ever becomes viable.

That would only be relevant to commercial-based optical spectrum providers. Gov mils have SAR (Synthetic-aperture radar) capabilities that would render that moot.

Can SAR penetrate steel hangar roofs now?

You mean the long range stealth bombers that are parked in enclosed hangars when not in operation?

Nonsense, spy sattelites were a thing when that program was developed. Even now it's pretty difficult for a sattelite to reliably spot, let alone track a flying airplane. (I'm making this up by the way, I have no sources or information and if any side did have sattelites with that capability, they wouldn't tell anyone)