I used to have a Normalize function under Colors->Auto in version 2.8 of GIMP. I recently upgraded to 2.10 and the function disappeared. This thread has one answer where the author attached a plugin contrast-normalize. I put that in the C:\Users\MyUserName\AppData\Roaming\GIMP\2.10\plug-ins directory but it didn't show up under Colors->Auto. Perhaps that attached plugin is not compatible with Windows 10 that I am on. Is there a way to to enable normalize on 2.10? If not, is there an alternate way in GIMP 2.10 to do the same thing that normalize in GIMP 2.8 was doing?
Looks like they removed that filter, probably because it wasn't working, or maybe not particularly useful or worth updating. This is just a pure guess. I don't know what their true motives were.
A possible work around is to install the G'MIC pluggin for GIMP, which has several Normalize filters you can try. I have no affiliation with this site or the plugin developers. See example below.
It's been a while since I used 2.8 so can't really remember exactly what the filter did, and I hardly ever used it.
Might help if you were to show and example of the effect you want. There may be other filters that could be used.
If above effect is too strong, maybe an auto levels adjustment?