DOOM Eternal is one of the few AAA games coming out this year that isn't going to include microtransactions. Director Hugo Martin confirmed this in a Facebook post, stating that players won't even have an in-game store to deal with. You pay your $60 and get the full game. Check out Martin's full statement on the matter below.
No store. Nothing you can unlock in Eternal with XP has anything to do with player abilities or content that would impact your game in any way. The only thing you can unlock with XP is cosmetics. These cosmetics have no impact on how you play, they just look cool.
Eternal is a $60 game, not a free-to-play game or a mobile game. We are giving you a complete experience with no store just like you'd expect. Unlocking skins with XP is a part of the experience if you care about that stuff, or you can completely ignore it and it will have no impact on your experience at all and it's all free.