Maybe it is on purpose. Pitch and Roll get zero level from gravity, but there is no absolute reference for Yaw (no potentiometer, no compass etc.). Also due this lack of reference Yaw is drifting. So maybe better control it on speed mode?
If angle control mode is used for Yaw, where is the zero angle? I think it will be on the direction the gimbal was started with, until it drifts. But maybe the new 'estimate angles from motor' helps on this and actually kind of references it to airframe...
Anyway, I have nothing against Yaw angle control, just thought there might be a reason it is not available.
BTW related to RC settings it seems LPF can be set to 0, but when reading back it is 3, it appears 3 is the minimum value of LPF.