"While we are not ready to announce specific dates, it is safe to say that we aim to release one new class every quarter.
We haven’t fully decided if we will charge for new and upcoming classes, but that is an option we are discussing. This isn’t our way of trying to bait and switch anyone. Fatshark has always prided itself on being very player-centric. We want to stay true to that in our monetization practices. We never intend to split our player base - you can expect in-game content, such as new areas, game modes, and missions, to remain free. We’ve learned a lot, and we think we have done a good job in Vermintide with our monetization of cosmetics and careers, and we are looking to that and other areas of feedback before we make a final decision."
Warhammer 40,000: Darktide - Classes in Darktide - Steam News (steampowered.com)
Fair to assume that classes will be paid content along with cosmetics, with maps etc. being free.