How about that?
If we drop the 2.x version, VTK will use the bundled pegtl library, as long as they don't have updated to 3.x.
IIRC I tried to disable it some time ago, but it didn't work out. So, IMO VTK needs this library, even when build without any USE flags set.
The test failure didn't seem to come up in bug 899470.
See also https://gitlab.kitware.com/vtk/vtk/-/issues/18151 for an issue to update to pegtl-3.