I actually installed Windows 98 on a Z790 DDR5 motherboard with a Realtek 8125 2.5Gbit LAN. The NDIS 2 driver initialized successfully the card in DOS, but it didn’t seem to work in Windows 98. Maybe I didn’t try hard enough, I gave up after a few restarts.

Anyway, for PCI-E Gigabit LAN in Windows 98, you should focus on Broadcom NetXtreme 57xx cards. I recently tested two such cards: a BCM95761 model from 2010, which didn’t work, and a BCM95722 from 2008, which worked perfectly fine. The model numbers make no sense - the 5751 cards are older than the 5722 ones, for example. So, if you’re planning to grab one from eBay, pay attention to the manufacturing date. The driver is from 2006, so the closer to that date, the better.

Btw, you can see these two Broadcom cards installed on Windows 98 (along a Quadro FX4500) on this video.

I have an Intel Pro1000GT in my 233MMX system simply because that's what I had on-hand. There are 98 drivers for it and they work fine, so that's another Gig-E option for 98 folks.

Support and speed are two different things under Win9x or XP 🤣.

Tweaking the TCP receive window largely fixes the speed issues under XP, the NT/2k/XP TCP stack is pretty decent actually. Win9x... well, it sort of manages to push packets over the network 😉

As for the use case for a 2.5GbE adapter - I'd say it's purely relevant for trying to install Win9x on hardware that already has one, doesn't have space for a second, more easily supported adapter and/or needs to dual-boot with something modern. Tbh, I'd just run Win9x in a VM on such modern hardware if I really wanted to, but others apparently relish the challenge of getting it to work on bare metal never intended to run it.