Movies in the 70's & 80's was it stereo or mono?

It would depend on the movie and theatre. Surround sound started in a big way around the mid 70's with Star Wars using two optical tracks (Dolby Stereo system). Some films were done in stereo prior to that using magnetic tape but this made the films expensive to produce. The Dolby Stereo optical system was the big breakthrough (two optical tracks gave four channel sound) because it was cheap to distribute.

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The reason I asked is that I'm about to rent a 1971 movie from Netflix and it states that it is in Dolby Digital Mono, whatever the heck that is.
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Right around the time Star Wars: A New Hope was released (1977), I went to see the premier of The Grateful Dead Movie at the Harvard Square Theatre. They had to bring in 4 stacks (1 in each corner), amps and a mixing console to do "four channel sound". Pink Floyd really took the lead in doing that in live concert setting. I saw them at Ivor Wynn Stadium in Hamilton, Ontario in 1975 with their full two stacks in the rear and two stacks in the front 360 degree Wish You Were Here, Dark Side of the Moon, Meddle tour.

This was an expensive undertaking, resulting in very limited releases to places that could accommodate the gear. Dolby Surround (pre-digital) on optical tracks revolutionized movie sound tracks, making it more economically feasible for theaters to convert to "four channel sound". I can still remember the opening sequence with the huge Empire cruiser passing overhead, and the sound moving from the back of the theatre to the front.