TiredDad posted a thread in here recently ( BalanceWatch a 6v6 project ) about his BalanceWatch 6v6 custom mode. After that I started looking to see if there were any custom servers up with 6v6 modes in general, and I found the OW1 Emulator custom 6v6 mode by ECKSDEE.
The work these creators have done is impressive and it’s been great to see players, new and old alike, have fun playing Overwatch.
Even if you never played OW1, I highly encourage people to try these out. You can even run it as a private game and fill it with bots if you just want to get a feel for it.
It’s been so much fun to revisit 6v6. ECKSDEE’s OW1 Emulator puts Doomfist back in DPS, returns the OW1 abilities to heroes like Cassidy and Mei and so on.
If you want to run the mode yourself, these are the codes for modes by TiredDad and ECKSDEE
Author: TiredDad
Game Mode: BALANCEWATCH 6V6
Game Code: 2SFET
Discord: discord.gg/JDBhMKH292
Description: OW2 heroes and abilities balanced for 6v6.
Author: ECKSDEE
Game Mode: OVERWATCH 1 EMULATOR V0.11.0
Game Code: KHTG0
Discord: discord.gg/myXGQk7h
Description: OW1 abilities and 6v6. They also have a Top 500 leaderboard and ranked mode you can join via their Discord server.
I found a third mode, also by Ecksdee. This one is called OW1++ Emulator it’s mix of OW1 and OW2 (so OW2 tanks are there, but Mei, Cass, Bastion, etc. are their OW1 versions, etc.)
Author: ECKSDEE
Game Mode: Overwatch 1++ Emulator
Game Code: YKMXH
Discord: discord.gg/myXGQk7h
EDIT:
ECKSDEE was running a server today. Normally the OW1 Emulator is 2/2/2, but they were about to play goats when I got in there. I filled Lucio and here are a couple replay codes if anyone wants to see the 6v6 chaos of goats with people from GM - Plat. 
Game 1: DFFZ8N
Game 2: 1HGSED
The slow-mo parts you’ll see is the anti-crash protection, pretty cool feature.