Right now, though there are already a decent amount of things retrieved, but this does not get NFS stats if they're available, which can be valuable for diagnosing potential perf related issues.

Essentially, the missing files are the ones presented also through the nfsstats and mountstats commands:

• /proc/net/rpc/nfsd
• /proc/net/rpc/nfs
• /proc/self/mountstats

Now:

cpuinfo.txt
meminfo.txt
mounts.txt
self/
  cmdline
  environ
  limits.txt
  status.txt
swaps.txt
system-uptime.txt

NOTE: main issue to watch out/test, probably, is that the current list seems to be unconditionally retrieved. So as those new files may or may not present, we need to make sure the support-bundle generation still works on system where NFS is absent.
UPDATE: so actually, the standard support-core plugins's FileContent class already handles this: if a file cannot be found, it will carry on and just put a notice that it cannot be found in that file in the zip. Which makes sense in the end indeed: the goal of that plugin being to gather as much things as it can in a prod env. It this fails fast at the first issue, it'd probably be useless.