So the Comms Officer and Science Officer roles were still there, they just got folded in elsewhere. More specifically, the Operations role. While not the dedicated Chief Science Officer, it served the bridge role of it and was responsible for numerous scientific experiments. Ops was also in charge of communications of the ship and coordinated all the bits and bobs.

Arguably, Troi also fills that role for Comms officer in certain roles.

As for TOS, they didn’t even have a tactical officer. Weapons were usually fired from either Helm or Navigation (which was also folded into Ops).