I figured not using Black Bull was just preference, yeah. In fact, muumu's rules as described in Japanese on his site specifically mention custom ("original") machines with D or E body:
muumu wrote: |
性能E及びDのボディのオリジナルマシンを使用しない |
(And as we've realized, Black Bull actually has an A body. I figured someone would've pointed that out if Bull were meant to be banned too.)
If you're wondering about other top players: Brave's formulation of the rules just says D or E body without mentioning custom, but doesn't mention Black Bull.
Rolly talks about side attacks but he didn't discuss banning of particular machines. If you look at his ASD and SOSS guides, he acknowledges that Black Bull is probably a better fit for the course than what he used, but he used Fat Shark as a personal choice/challenge. So it sounds like Black Bull wasn't banned or anything like that. Rolly has also done Great Star runs of MCTR and MCSO, and that machine's MTS is of comparable strength to Black Bull's.
I know of two players who used Black Bull without ever using side attacks. ColonelFXGX (BFGX) used it for his SOSS and ASD times at one point (although he may have improved both with Gallant later). foxu also seemed to favor non-side attack runs by choice, despite using Black Bull; Nicholas Mihaila (TAO) indicated as such in this PRSLS video description (I'm assuming he knew the historical context as well as anyone):
Nicholas Mihaila wrote: |
This run represents an upset in GX history as Foxu beat the current world record with a non-custom machine, a feat which was especially impressive because of his decision to not use side attacks. |
So overall, I don't think Black Bull was ever banned by muumu's rules, which I'm assuming were the rules that influenced most or all of the players mentioned above.