For advanced techniques, I agree, but the same probably applies to advanced bicycling techniques. Basic handwriting is probably not something you would forget, just like basic bicycling.
Riding technical trails on a MTB. Without regular practice you lose a lot of pace.
Street or park BMX tricks (tail whips, flairs, flips, grinds etc). These require skill plus confidence. Without regular practice the confidence won't be there to attempt them.