With OAuth support there will come database changes which requires a major version change. So yea, once we release v1.17 (OAuth support) 1.15.x will be no longer supported. Since we aim to have OAuth support by the cutoff of May 2022 you should plan for 1.15.x support to drop on or around that time.
Yes, 1.15.x is the last series to support PHP 7.4 and below. Everyone should be moving to PHP 8.0 soon anyways as PHP is dropping all support (even security support) for 7.4 in November of this year.
We will be doing testing with PHP 8.1 on 1.17.x so that series will eventually fully support PHP 8.0 and 8.1. As I recall there was only one issue or so with PHP 8.1 in 1.16.x so I don’t think it would take much.
As far as the 3rd party mods go, that's the responsibility of the maintainers of the mods to keep up to date with PHP and the codebase.