(21 May 2022, 3:14 pm)streetdeckfan wrote Depends who you ask though.
MG said during one of the Facebook lives that the StreetDecks were fine on the X21 and they were only removed due to 'operational reasons', presumably needing somewhere to dump the B5s.
However, Dan has said they were removed due to their unreliability, although I'm pretty sure he doesn't comment on here in an 'official' capacity.

I do think, having used them extensively, the B5s are a better fit to the X21 than the StreetDecks were, although I don't think any of the new 4 cylinders have ventured down these parts, only the 6 cylinder, which seemed to cope pretty well.

They were removed to the following:
- 6301-03 - Various
- 6304-07 - X5/X15
- 6331-33 - X45/X46

Also bearing in mind some of the original batch of 30x (OM934) were going to be for the X21 too with the 15 minute frequency increase pre-covid. But 9x got diverted to Oxford. In return, coaches were available for the X9/X10 so in turn, the B5TL's got allocated on to the X21.