I had this issue with some systems, but not all. Since these
systems have been upgraded from Jessie to Stretch, not all of them
have been changed by apt to what I call full-systemd.|
After being bitten by this issue, I have learned how to predict which will fail and which will not. If the output of 'file /sbin/init' is "/sbin/init: symbolic link to /lib/systemd/systemd", then apache2 will start after the upgrade.
However, if the output is similar to "/sbin/init: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 2.6.32, BuildID[sha1]=4a7d2a34f4f122bf7c614e277f521789f675764b, stripped", then apache2 will fail to start.
So currently, the apache2 upgrade absolutely requires the systemd init system, but not all systems have that in place. For reasons known only to apt, the Stretch upgrade path has not led them there.