There was a decision made in coreboot release 4.16 that future releases would not include any AGESA boards.

This was because LEGACY_SMP_INIT and RESOURCE_ALLOCATOR_V3 was deprecated and nobody did the work to integrate PARALLEL_MP and RESOURCE_ALLOCATOR_V4 into AGESA boards.

In response to that, I tried my best to port the HP T620 board from a working 4.18 fork[1] to use the new codepaths. This work for FAM16h is incomplete but nearly there.[2]
I believe dasharo[3] (3mdeb) is also working on FAM14h and possibly FAM15h as well as Asus KGPE-D16 to upstream their work.

More recently, a member of the coreboot community said he wanted to do some work to bring back some AGESA boards to master.[4] Perhaps he can reuse my bring-back-agesa branch.

If this work is completed, there is potential for more gnuboot compatible boards supported in coreboot master.

[1] http://git.zammit.org/coreboot.git/log/?h=hp-t620-418
[2] https://review.coreboot.org/q/topic:%22bring-back-agesa%22
[3] https://dasharo.com
[4] https://mail.coreboot.org/hyperkitty/list/coreboot@coreboot.org/message/7OOTVML4LNC63DBPUODYRCLRHPCAFROP/