I assume that after "Channel to selection" you bucket-filled the selection with I white on a black layer.
On Gimp 2.8 the behavior is as you expect(*). 2.8 works all over with 8-bit gamma-corrected values (so that there are more values to describe the dark tones).
In 2.10 things are different. Internally all computation is done in floating-point, using non-gamma-corrected values. When you copy the channel to a selection the plain channel data is copied, and since the selection is not really an image, there is no reason to gamma-correct it.
There is still a way to apply this gamma-correction, though. Assuming you have a selection from the Blue channel:
- Create a layer filled with black
- Add a new layer, and bucket fill the selection with white (you get the "too-light" result)
- Change the blend mode of the white layer from
Normal (legacy) and you get your expected result (basically the "legacy" blend modes skip the gamma-correction).
(*) "Correct" is a bit strong here. There are plenty of ways to create monochrome images, all as a valid as the others.