1. These boards will be gone for the people who check the "mainboards
supported by coreboot" and see only the "new Intel stuff". This hinders the coreboot community growth around the "gone boards", and also of the coreboot community in general: the fewer boards are supported by coreboot, the more difficult it is for a potential user/contributor to find the supported board and join us.
For the record, we have removed Intel boards from the master branch in the past - See 4.11_branch. This was for boards that used FSP 1.0, including popular Baytrail Atom and Broadwell-DE platforms which are still widely used today. This ensures that those platforms continue existence on an easy-to-find stable branch where one can reasonably expect to check out the code and have it work. Checking out the master branch only to find out that it doesn't work and then bisecting years worth of commits is a poor user experience.
Perhaps we should follow the 4.11_branch example and do something similar with old AGESA boards? Boards which are forward-ported and tested can stay (or be re-introduced) in the master branch, of course.
Many of the AGESA platforms in the list Arthur provided are ~10 years old. Some are clearly obsolete, like the Gizmosphere boards that have not been in production for years and whose manufacturer is defunct. Others like the PCEngines APUs should be more readily available to test and have developers able to spend some time forward-porting the necessary bits.
Lastly, I'll mention that there is an active crowdfunding effort to re-upstream KGPE-D16 support: https://github.com/osresearch/heads/issues/719. There's clearly a lot of enthusiasm with that board, and 3mdeb is already porting allocate v4 to it. Perhaps enthusiasts for other boards can piggyback on this effort and leverage some of their work to bring other boards up to date.