Bug 1515579 removed some necessity for them, but kept a hack mentioning bug 1516228 and bug 1516253, but the former was fixed since then, and the latter is presumably fixed by bug 1524396.
However, we need to ensure that the change from bug 1537669 keeps working (cf. https://phabricator.services.mozilla.com/D24326#716073) and other things that might indirectly rely on $PATH containing the toolchain search path.
Bug 1537643 is another blocker.
And even though we do pass the full path for cl.exe to midl.exe, it still needs PATH alteration for mspdbcore.dll, so that makes bug 1537641 another blocker.
With the patches from bug 1537643 and bug 1537644, I was able to get successful win32 and win64 builds with the $PATH-altering hacks removed... but not for win64-aarch64, because armasm64.exe has a dependency on the x64 msvcdis140.dll.