There is one tool of which output changed significantly between GIMP3 and GIMP 2.10. This is the equalize command (histogram equalization: [Colors] > [Auto] > [Equalize]). This command is very useful in restoration of faded film images as I appointed in the post below.
Previously, it worked in a perceptual RGB with sRGB TRC applied, but in GIMP3 it works in linear RGB, which resulted in the output becoming much brighter.
original image
equalize applied image in GIMP3
I suppose the reason for this is as next. In 2.10 the histogram which this command uses was calculated on a perceptual RGB basis, and the subsequent reassignment of values is also done on a perceptual RGB basis. However, in the current version of GIMP3, although the histogram calculation is still done on a perceptual RGB basis, the reassignment of values is now done on a linear RGB basis. If the allocation is done on a linear RGB basis, the histogram calculation should also be done on a linear RGB basis. So as a result of this discrepancy, the TRC is multiplied twice.
Therefore, I created a plugin that will return this to the same brightness as the 2.10 output. If you apply this plugin to an image that has already been equalized in 3.0, it will return to the same output as in 2.10.
This tool simply decodes the sRGB TRC. If you apply it to an image with the TRC applied incorrectly twice, it will change it to an image with the TRC applied correctly once.
the result of applying this tool
This tool can be downloaded from the link below.
Once installed, it will be available to select from the [Colors] menu.
There may be other tools besides equalize that may have similarly brightened output, but I think they can be used as well. However, currently, they can only be used with RGB color images.