It looks like AntPool recently signaled BU support.
Yeah but the problem there is signalled support is a little different to what core thinks.
Although we like to think that bitcoin isn't under the control of anyone, core thinks they control it and act as such.
Someone would have to actually push a commit to core for BU and then core would have to actually accept it and put it into a release version.
Why would they do that?
Yep bitcoin is not decentralised.