Comment 4 for bug 1867537

Oh, I just found the file. It has a different name than suggested in the changelog:

/usr/lib/sysctl.d/protect-links.conf

And silly me didn't of just checking what sysctl returns for those values set in there. It's all good:
$ sudo sysctl fs.protected_fifos
fs.protected_fifos = 1
$ sudo sysctl fs.protected_hardlinks
fs.protected_hardlinks = 1
$ sudo sysctl fs.protected_regular
fs.protected_regular = 2
$ sudo sysctl fs.protected_symlinks
fs.protected_symlinks = 1

So, from my POV the debsums error, while technically correct, is a non-issue.