Would be news to me, it would also be incredibly stupid by Motability – they get value for free if you add options, so why would they prevent you from doing so.
I know for a fact that in Nov 2021 it certainly was possible to add options, since we did.
Prior: SEAT Ateca Xcellence Lux 1.5 TSI DSG MY19, VW Golf GTE PHEV DSG MY23
Current: Hyundai Ioniq 6 Ultimate
Next: we'll see what's available in 2028.