alt Hacker NewsThere is also the Libreboot which may be of your interest. What I see is that there is a lot of development on Coreboot but it is indeed mostly driven by companies working on their latest platforms. There are not many contributors working on older platforms. Though in my experience the response to patches is relatively quick.
I had the exact opposite impression, there are a lot of user contributions working on old platforms, and even specific platform communities. For example, you have 15h.org which is a community dedicated to AMD Bulldozer / Piledriver generation, and they managed to fix a few things that were left broken in Coreboot. I also saw an influx of users working in Sandy Bridge / Ivy Bridge / Haswell era Intel platforms with an occasional Skylake. The problem is, precisely, that most of the user contributions are for old platforms that nobody that does NOT currently owns those will want to touch if you're purchasing new. I would have been very enthusiastic about that work if it was done about 10 years ago or so, nowadays they seem to be hobby projects that don't advance anything forward.
There do exists individual users that work on newer stuff, after the MSI Z690-A / Z790-P there were 2 people that did ports for H610 since at that point the platform was supported and they only had to port the Motherboard. However, they are rather rare, since the kind of user that Coreboot seems to attract are from the privacy and security anti-ME/PSP crowd that don't even want to use newer platforms, which is the exact opposite of my view.
A lot of the work companies does may be just for their internal systems or reference platform and these may not get to the hands of end users, ever. I still recall Phoronix articles covering Coreboot releases always taking a jab at the new supported Motherboards until the MSI Z690-A DDR4 port from 2022.