Well it isn’t exactly a lobby, but I found a website that has someone hosting Halo Reach, Halo 3 and Halo 4 and 5 custom games! It’s called HaloCustoms, it seems like not too many people use it but I’ve signed up for it and there is a 3 hour lobby opening up on the 28th this month! So far there is only 4 (potentially 5) of us signed up for the lobby, but would be sweet to get more folk in! Check out the website and see if you wanna join! 
You’ll see my name RSVP on the Halo Reach lobby!
I am also not moderator or anything of this website so I am not promoting my own stuff just wanna see if anyone wants to sign up as members and join me on some custom games!