EU server, 18:00-18:20 CEST, 20th September.
Made an offer in General Chat to give softcore, seasonal 1-70 boosts.
Received a whisper from someone asking for a boost.
Whisper showed without a leaf, indicating a non-seasonal hero.
Viewed player’s profile, showed the hero was softcore, seasonal.
Invited player to party but unable to start game with message saying that it’s not possible because you can’t mix seasonal and non-seasonal players.
Dropped party, started a game, tried re-inviting the hero, said I couldn’t invite for the same reason.
Also received a whisper from another player that said they were able to offer hardcore, seasonal boosts. That whisper showed without a skull or leaf icon, indicating the whisper was from a softcore, non-seasonal hero. When viewing their profile, they were on a hardcore, seasonal hero.
My Battle.net launcher shows one of the players on my Friends list is playing Diablo III, in town, at T16. In-game, my Friends list shows them as Mobile, i.e. not in game.