First form of TouchID appeared on the iPhone 5S. https://en.wikipedia.org/wiki/Touch_IDIf Apple names it the 12S, then we can forget about the return of Touch ID in 2021. An S designation merely suggests a bump in specs, not the introduction of new features.
Generally speaking "S" phones was always about refinement inside of the design, but that doesn't mean new features weren't added. Usually what didn't change was the body, but inside of the body everything was up for alteration.
Regardless, I don't give a whole lot of credence to rumors regarding under-screen TouchID. Unless Apple can come up with a version that isn't defeated by a tempered glass screen protector, I doubt they'll utilize it. I find it more likely that they'll do a power-button based touchID - much like the new iPad Air. But it might get pushed another gen unless it's "perfect".