Resident Evil Outbreak File 1 and 2 were the last "classic" style RE games that came out before RE4. They were also multiplayer-focused online co-op games... on the PS2. In 2004. I said "still playable" in the title but honestly, even for people who were RE fans back then, it's probably more like "actually playable for the first time." The fan server is also definitely not some new thing- I think it's been around for like a decade at this point.
If you've ever wondered what it would be like to play an old school survival horror game online with friends, these are the games for you. They're also pretty hard though, even if you're someone who can breeze through every other game in the franchise.
The tutorial I linked goes over through this stuff as well, but you'll need the following to play:
- An account on the fan server forums (free)
Legitimate American and Japanese copies of both games, ripped to a PC so you can run the required patchers to patch the Japanese isos into Englishlol just kidding, just grab English-patched Japanese isos off the Internet- Memory Card files with the required online data on them (linked in video description)
- PCSX2. As long as your machine can run PCSX2 and has an internet connection, something like a Steam Deck should work fine too (you can also apparently play the game online using an actual PS2)
You'll probably also want to wrangle some friends to play the game with you, which in my experience is the hardest part 
i played this in single player so many times and loved it a lot. always wanted to try it multiplayer but i didn't had a good connection or friends to play with. nowadays i have no time but i would love to try it someday. it always amazes me how much the community can achieve when there is dedication even if companies really makes it harder for everyone
I had never really played Outbreak too much back in the day since it just didn't appeal to me as a single player experience even though I loved all the other Resident Evil games. As soon as I heard about the fan server I knew I had to try it out with some friends.
From what I understand the fans reverse-engineered the server by analyzing the packets the PS2 was sending out to the official game servers, not that I really know what that means
It was just unbelievably cool to get to experience the PS2 online experience 20 years later and I have nothing but respect for the sorts of people who make things like emulation and fan servers possible 