I swear it wasn't this hard 15 years ago 
I would chalk it up to being 
er but I can still jump into a random game of Gears of War and do okay, so who knows. I guess the skill ceiling in the old Halos is just much higher due to the battle-hardened diehard audience left playing them?
Not on the PC, it isn't, despite the constant calls from the PC userbase to add it in. I actually think the MCC team has done a great job with the game. They've gone above and beyond making a simple barebones PC port and added all kinds of fun additional content to the game, and they're still quietly chugging away at it. Just a shame the matchmaking can be so brutal.
There is the Custom Games Browser too, but most of the games on there seem to be weird Forge minigames and modded gamemodes. I just want a casual Slayer or Big Team Battle match from 2008
I guess you could try hosting your own game on the CGB but I haven't given that a shot.