I think 650c is a popular size for wheelchairs and disability racing tricycles. There are still plenty of Time trial bikes around using 650c as a front wheel, too.
I would like it if one day that UK time trials will have classes for
modern aero TT
modern road
classic metal road
Classic carbon road
classic metal time trial
Classic carbon time trial
I have no idea what date to do the split at. There should also be more classes for ages, sex, tandem, trike etc and have all of it happen over 1-2 days on an interesting motor racing circuit for safety with lots of prizes, trade stands and music/bbq overnite. Have a road races too while we are at it for juniors, women and men.
The Americans do this and we really should be doing it because our roads are dangerous and there is too much separation and barriers of entry in the sport. Something needs to change as we have fewer and fewer events and competitors, licence holder, and club membership numbers are dropping fast.