Yet rotary phones were still very common in the 80s, and my grandparents still had one in the early 90s. So people born in the early 80s, which qualifies as millennial, very likely used them.
Until 2000-something (and they might still have it) the Harwich MA public library had a rotary phone that patrons could use to make calls. The typical use case was middle schoolers who's parents forgot to pick them up (the middle school ran a bus to the library) when the library was 30min from closing.