Before boarding my District line train at Kensington Olympia today I
dutifully swiped my monthly Travelcard Oyster card on one of those
often-hard-to-find validators that says "Pre Pay users Please touch
your Oyster card on the reader".

Many posts in this group mention that actually, ALL Oyster users have
validate their cards to avoid an unresolved journey. My question is,
why did LU/Transys print those misleading Pre Pay users only notices?

They seem to be written for a period when (i) Travelcard Oyster card
users don't have to worry about swiping in and out, and (ii) Pre Pay
is in use. AFAIK these 2 conditions never existed at the same time.
Was the original plan to keep Pre Pay disabled on Travelcard Oyster
cards, but there was a late change of plan?

Dominic