If you have a general understanding of the characters, you could jump in and have fun. It'd definitely be worth it to see the first two Deadpool movies and Logan first for better context, but I'd say it's easy to follow. From the Marvel side, the Loki series provides background on a key organization in the movie, but don't sweat it if you haven't seen that either (But it definitely is good)
The whole thing is basically a capstone to the Fox superhero universe:
Blade
Blade 2
Blade 3
The Blade films are from New Line, a Warner Bros company, which is why it wasn't included in the mid-credit montage. It's included because many people (including myself) count it as the first modern demonstration that Marvel movies could be good and it was the first building block for the MCU - a proof-of-concept if you will.
Just the Deadpool movies, to get the majority, probably the Marvel Cinematic stuff to get some other inside jokes, but that's a waste of time and not worth the effort, and if you want deep cuts, read the Deadpool comics. That's how they make these movies; there's stuff for the people that only watch the directly involved movies of the precise franchise, plus stuff for the total nerds. That way almost anyone can get something out of it. I like Deadpool, so I say watch the other two and then see this one. If after the first one you decide you don't like it, don't bother with the rest. It's really that easy.