Same here, I grabbed it on GoG and have been playing fine on steamdeck through Wine and as a “Non-steam game”.
It is odd that the game is marked as “Mature sexual content” and thus unavailable in some places as there isn’t actually any explicit nudity in the game.
Perhaps the when setting up the store the devs didn’t realise that notifier is intended more for explicitly pornographic games and not just fan service stuff.
Games like Senran Kagura for example which I would say has comparable levels of fan service and skin shown are still freely available as they do not have a “Mature Sexual Content” tag.
I hope the TurretGirls devs take note of this and see if it is possible to safely alter the Steam listing as it’s content shouldn’t be an issue.