I think it depends on how far they have transitioned and if they are just having sex/building a relationship. If you have some obvious signs that you are transitioning from one gender to another (this mostly MtF in my mind) then a heads up may be good. This is primarily for safety as some people may act violently when it is discovered.

If you and the other person are just hooking up or having a one night stand and there are zero plans for the relationship to progress further than that then I see no reason to overshare (except for my statement above). there is a lot about people you won’t know in this situation and this is just one more thing.

If someone is looking for a relationship or one does form expectedly or unexpected from a hookup then yes I think the other party should be informed early. People have a view on what they want from their life and a transgendered partner may limit some of those and it would be better if that can be think it over before things become too intangled.