This happened as part of a recent procps update (2:4.0.4-6). It's documented in the changelog [1] but it didn't appear as a pop-up during 'apt upgrade' and there wasn't the usual dialog asking to delete/remove config files. I was surprised that an important file would be deleted without warning. (I'm not saying whether the overall intent to have files in /etc/sysctl.d is right or wrong. And it could be that I missed a note a long time ago telling me it was depreciated.)

Restoring the /etc/sysctl.conf from a recent backup fixes it as the file is still honoured.

Just posting here in case others experience the same.

[1] https://metadata.ftp-master.debian.org/ … _changelog
procps (2:4.0.4-6) unstable; urgency=medium
  * Remove old conffile sysctl.conf Closes: #1076352 [2]

[2] https://bugs.debian.org/cgi-bin/bugrepo … ug=1076352