Ahhh, this reminds me of the same custom clients that were made for call of duty modern warfare 2 called iw4x. Used to play it a lot many years back, I remember even some indian servers being hosted on it and the servers were quite filled up back then surprisingly.
Afaik, the team behind iw4x renamed themselves to X labs and now also created modded clients for cod ghosts and advanced warfare as well. Only disappointing thing is other than mw2, the rest of the games playerbase is quite dead, aw was fun for a while to play against bots but it got old fast.
Ghosts and mw2 seem to have the same story with only the trickshot servers being populated. Black ops 2 also has a custom modded client called Plutonium and I think that's by far the most popular modded client because of the popularity of black ops 2 itself. Hell if I'm sure there was also a huge tournament played on plutonium and even the big streamers like dr disrespect participated it lmao. Crazy to think it would have got this big from the plutonium team having just a mw3 client.
The Plutonium Project is a CoD mod that aims to improve upon your favorite games.
plutonium.pw
Completely forgot to mention, I used to host a few isnipe servers as well on iw4x, can't seem to remember how or what the name was tho but it worked really well and was populated