I've been running Jessie without systemd on my laptop for a couple years now. Never had to worry about packages not working, except for vbox not building it's driver. But that's probably some issue with backports, and I don't care enough about vbox to actually look into it. Systemd is installed though and apparently used for udev and logind.
Edits: I should review before posting.
so yeah, there is practically no way to escape from systemd in Debian using vanilla packages.