@Niels_Dekker et al thanks for looking into this, submitting a PR, and creating this thread!
I have been using C++17 with ITK for quite some time, and it does have very nice advantages.
There is not a ITK/toolchain usage scenario I am aware of where the toolchain cannot be upgraded to support C++17 when upgrading ITK.
When upgrading the required standard version, we should also make a note in the ITK Software Guide and describe the associated minimum compiler versions here: