How to safely eject a disabled USB drive?

FWIW, the safest thing would be to shut down before removing the drive.

I had the message of the device being disconnected for power use and the drive was not visible in finder and not on the desktop. Yet when I then pulled the plug, the message about proper ejection still came up.

Well, then it mounted the drive in the process of disconnecting it. Regardless, it only ever matters if you are writing to the device at the time you disconnect it. Doesn't sound like that was possible in your scenario since it was disabled.

macOS does write caching on removable devices just like internal devices. So, if you had written data to the drive, it is possible the changes could still be writing when you disconnect the drive. That is all the warning means. The warning has no idea whether it was still writing, just that it didn't get the opportunity to check.

If it is still writing the cached files when you disconnect, you could lose data and possibly damage the drive (but I think that would only be for spinning hard drives). If it isn't still writing, then the warning is just an annoyance.

Also, it doesn't wait until you eject to write the cached files. It writes them pretty much immediately, so it would only likely be a problem when writing a very large file to disk, then immediately pull the plug after Finder says it completed writing.

when it's not fully connected it is always safe to disconnect

So the warning to safely eject before removing is then just a bug?

You should only see that message if the volume was actually mounted.

In your description of the problem, your drive isn't mounted.

If you get that message when you disconnect the drive, then it is mounting and not disabled.