Well, not a short read but start at top of this thread and work down! 
To clarify this has no affect on photogrammetry or TIN areas (they are mapped in high resolution by default), we are taking about the 99% of the planet that is not these
Another Wishlist here with before pictures: Provide a MSFS 2020 SU15 / Legacy Option
But in summary:
- Default aerials were MUCH better prior to aligning to 2024. At same time they lowered quality of default they removed support of a solution that had 4x increase in the resolution of aerials which was available via a 3rd party. So basically couldn’t have our cake nor could we eat it. Especially with the latter, the difference is rather a 10x decrease in the experience
- Heightfield was aligned to 2024, so now many areas that have cliff faces or vertical drops look like smeared oil paintings.
- TLOD curve was tweaked so now not as good out to horizon as it was prior. The world seems softer and smaller.
- Some have reported popping in of terrain has massively increased.
That’s the big 4. Would not have been so much of an issue if Asobo had done anything in last year to try and recover or revert these, rather we get comments of it not being intentional and empty promises in live streams about one day working on a “best of both worlds”.
If we don’t stand up to the true injustice to the community that was su16, it will show there is little reason for Asobo to be concerned when it’s 2024’s time to face similar “non intentional” downgrades when the next sim version is released.