Call of Duty: Modern Warfare released to acclaim from both fans and critics in October of last year, however, a game of its scale is bound to have some issues. One of the issues that fans have taken to Reddit to complain about is the sparsity of custom loadout slots used in multiplayer (via VG247). Just recently, on the very same Reddit forum, Infinity Ward's art director Joel Emslie fired up his developer-linked Reddit account to respond to the feedback, stating the following:
"You and everyone else that has commented on this are right. Its in mainline back at the studio and weve been tinkering with it. Im not sure which update its in but I think its next. Believe it or not it can cause horrible bugs if not tested properly. Im not certain about all this so don't get fired up on me if Im wrong. Since I've commented here Ill stick with the issue at the studio."
So it seems additional slots are being worked on. As for when that'll happen, we don't know quite yet, but we will keep you updated when more information is shared.