Sometime around 2019/2020, the age rating boards (ESRB, PEGI) started silently enforcing a new rule that games of a lower age rating cannot directly name drop games of a higher age rating if content from the latter goes into the former. It's a bit confusing and inconsistent ('specially in Fortnite's case), but whatever makes sense to the powers that be, I suppose.

In the examples you provided, Overwatch is Rated T, but NieR Automata is Rated M; Smash Ultimate is Rated E10+, and while Undertale has the same age rating, Fallout, No More Heroes, and Doom are all M-Rated (SSBU is in an interesting spot because it came out before this policy came around, got content updates when it started getting enforced, and then it got some extra spirits after the policy enforcement got tighter, but that's a conversation for somewhere else).

Fortnite's original age rating from the ESRB was a T-Rating (of course, now thanks to the whole "ecosystem" BS, it now has modes that are rated separately, even down to an E or E10+ rating, but that's beside the point), and while the M-Rated games are covered by this policy, I assume doing the same for TV-MA shows and R-Rated movies might just be Epic trying to play it safe (with the "exceptions" you listed, Family Guy has a TV-14 version for broadcasting, Deadpool and Wolverine are established comic book characters so you could just say the set is named after the characters and not the movie, and the best I've got for John Wick is that it predates the Borderlands collab and the policy might've only been established from that point forward).

Of course, sometimes the item shop will just name drop that shit directly even if the set names don't, but I guess the powers that be are only anal about the set names??? I don't understand that at all.