Was it ever just flagged as a bug, or some intentional planned obsolescence on the part of Apple?

Mine was out of support by the time I noticed. And for whatever reason I was in the vast ~~majority~~ minority of people who didn't think buying a new external NIC was an acceptable solution, like that other guy who replied to me here.

May just make it a server for plex or something.

Exactly what I was going to do with it when I moved it to somewhere wired. I now instead have a Synology Diskstation that can do hardware encoding/decoding. I still use the Mini, but after the PTP thing I wiped the disk and run Windows on it (for Lightroom Classic, and honestly the Gb would still be welcome with the media stored on the NAS, and I use Synology Drive to sync the Catalog folders)

I think there are some workarounds to get past the “supported OS” thing

If I remember correctly, with the first OS update that had the downgrade, you could roll back to the previous OS version (or boot Windows or Linux) and get it back, so it was just in the driver. But then at some point that stopped working too, so maybe an on-chip firmware update had been applied too. I tried store-bought and home-made Cat 5e/6 cables, different switch ports, hot-swapping different computers to confirm they would negotiate to Gb, and even loading the vendor OEM drivers and utilities after installing Windows.