This is how I managed to use the Nostalgic Neg profile on my x-t4 files. I think maybe it will help some other fujifilm photographers. So, this is what I did. Just remember I’m not a computer guy, it was a bit of a lucky guess.
First, find the “Nostalgic Neg” .xmp profile file for the X-T5. On macOS right click on the Lightroom Classic app and select “show package content” Then go to Contents—>Resources—>Settings—>Adobe—>Profiles—>Camera—>Fujifilm—>Fujifilm X-T5
Copy this file elsewhere, and open the copy in a text editor (I used Brackets).
Replace the line crs:CameraModelRestriction=“Fujifilm X-T5” by crs:CameraModelRestriction=“”
Also, change the name of the profile on line Camera NOSTALGIC Neg. Replace it by Camera NOSTALGIC Neg 2 for example.
Then save the file, make sure the extension remains “.xmp”. Finally import the profile in Lightroom via the file menu, and it should work.