Technical debt goes hard, I had a discussion with a facilities guy why they never got around to ditch the last remnants of token ring in an office park. Fortunately in 2020 they had plenty of time to rip that stuff out without disturbing facility operation. Building automation, security and so on often lives way longer than you'd dare planning.

There's plenty of use cases for small things which don't need any sorts of speeds, where you might as well have used a 115200 baud serial connection but ethernet is more useful. Designing electronics for 10Mbit/s is infinitely easier and cheaper than designing electronics for 100Mbit/s, so if you don't need 100Mbit/s, why would you spend the extra effort and expense?

If you worked in an industrial setting, legacy tech abounds due to the capital costs of replacing the equipment it supports (includes manufacturing, older hospitals, power plants, and etc). Many of these even still use token ring, coax, etc.

One co-op job at a manufacturing plant I worked at ~20 years ago involved replacing the backend core networking equipment with more modern ethernet kit, but we had to setup media converters (in that case token ring to ethernet) as close as possible to the manufacturing equipment (so that token ring only ran between the equipment and the media converter for a few meters at most).

They were "lucky" in that:

1) the networking protocol that was supported by the manufacturing equipment was IPX/SPX, so at least that worked cleanly on ethernet and newer upstream control software running on an OS (HP-UX at the time)

2) there were no lives at stake (eg nuclear safety/hospital), so they had minimal regulatory issues.

There is always some legacy device which does weird/old connections. I distinctly remember the debit card terminals in the late '00 required a 10mbit capable ethernet connection which allowed x25 to be transmitted over the network. It is not a stretch to add 5 to 10 more years to those kind of devices.

This hasn't mattered in 20 years for me personally, but in 2003 I killed connectivity to a bunch of Siemens 505-CP2572 PLC ethernet cards by switching a hub from 10Mbps to 100Mbps mode. The button was right there, and even back then I assumed there wouldn't be anything requiring 10Mbps any more. The computers were fine but the PLCs were not. These things are still in use in production manufacturing facilities out there.

Some old DEC devices used to connect console ports of servers. Didn't need it per say but also didn't need to spend $3k on multiple new console routers.

Was an old isp/mobile carrier so could find all kinds of old stuff. Even the first SMSC from the 80s (also DEC, 386 or similar cpu?) was still in it's racks because they didn't need the rack space as 2 modern racks used up all the power for that room, was also far down in a mountain so was annoying to remove equipment.

Things I have found that only do 10mbit:

Old CNC equipment.

Older Zebra label printers.

Some older Motorola radio stuff.

That SGI Indy we keep around for Jurassic Park jokes.

The LaserJet 5 thats still going after 30 years or something.

Some modern embedded stuff that does not have enough chooch to deal with 100mbit.

APC UPS SmartSlot network monitoring cards. Only the very newest support 100Mbps....