Sessions don't work on stretch, with redis giving this error: MISCONF Redis is configured to save RDB snapshots, but is currently not able to persist on disk. Commands that may modify the data set are disabled. Please check Redis logs for details about the error.
The redis log says Failed opening the RDB file redis-db.rdb (in server root dir /srv/redis) for saving: Read-only file system. The file doesn't exist; /srv/redis is drwxr-xr-x redis redis and redis-server runs as redis:redis.
Interestingly if I just start it by hand via sudo /usr/bin/redis-server /etc/redis/redis.conf then it works fine (although MediaWiki sessions still don't).
Apparently the systemd file for redis sets everything to readonly and /srv/redis needs to be whitelisted via ReadWriteDirectories (an undocumented service directive - welcome to systemd). That also seems to fix the performance issues.