ipad and iphone still do not mirror on MacOS tahoe 26.1 when an screen recorder is on. Mac Screen goes black.

Since it worked in Sequoia, but does not work in Tahoe, more likely that it’s a bug than a privacy policy. Either way, it is unlikely that Apple will divulge which it is.

Bugs can be reported to Apple here:

Feedback - macOS - Apple

All you can do is re-report the problem under 26.1. Bug fixes are prioritized based on number of user reports, and unfortunately, this would seem to be a fairly niche issue – I doubt many people use this combination of features routinely.

I wouldn't recommend it. Is this particular functionality absolutely critical for you? If so, downgrading may be your only option. Reverting to Sequoia is not simple. You need to boot into Recovery OS, erase your internal drive, then install the older macOS, then restore from a backup made prior to the Tahoe upgrade. Even then, the firmware on the Mac will likely have been updated, meaning potential conflicts and a possible need for DFU mode and a Configurator revive/restore, which would need to be done via another Mac connected to yours. If that sounds complicated and risky, then you're assessing the situation correctly.