There are two components to this. First is the sound unit on the projector. It has to be able to read the stereo soundtrack. Sometime in the early 80’s, the last projectors that required a mono print either vanished or were no longer supported. Theaters could upgrade the sound unit and not the speakers. Many theaters could play stereo soundtrack but only had mono speakers. Of the 15 screens in my city when I was going to movies a lot (mid 80’s to late 90’s), at least 9 only had a lone mono speaker but they all used the same stereo prints.
Once you have the sound unit that can read the soundtrack off the film, you can play it as mono, stereo, or surround depending on what other hardware you had. Mono and stereo were mainly amplifiers and speakers. Surround required the surround sound processor to decode the 4 tracks.