Why is a TT national champion not allowed to wear his jersey in TTT According this source, Wout Van Aert is not allowed to wear his new TT belgian champion jersey in TTT of the tour the france.

I can remember days where this was allowed. Why and when did this change?

Source
See rule 1.3.068 of the UCI regulations: "The national champion in the individual time trial is not authorised to wear the distinctive national champion’s jersey during team time trial events."

I do not know when this particular rule was passed but it does not appear to have happened in 2019 so it must have been earlier.