I had observed for a long while Nim was stuck at 1.4 in Arch Linux, but researching it further I found the lack of check-summable releases or pgp signed tags or releases since that time has made it genuinely impossible to update any further due to the security requirements imposed by the packaging guidelines:
https://wiki.archlinux.org/title/Arch_package_guidelines#Package_sources
This isn't a problem for the AUR nim-git package, or for Debian packages where a source package is created and stored, but the Arch system requires a download that can be verified somehow to check for mitm or data corruption issues. Having the build script download csources_v1 head from the git repo may be convenient for many situations but is unacceptable for creating official repo packages. Ideally there would be something labelled as being for the 1.6.4 release, and probably it would be helpful for Nimble too, although there are some releases and tags there, but not that recently, so I wonder what is intended to be used alongside Nim 1.6.4 there as well, Nimble is in the same PKGBUILD. All the AUR Nim packages I tried seem to build fine with nim-git recently except nimsynth-git which hasn't built since 1.0 anyway so there appears to be no reason to keep it at 1.4 and that's likely to make it increasingly difficult for Nim packages in future to maintain backwards compatibility if it can't be upgraded.
https://archlinux.org/packages/community/x86_64/nim/