It's always a bit dicey to try to predict Apple; they have a long history of zigging when folks thought they would zag.
However, my belief, based on High Sierra betas, is that APFS will not be mandatory in High Sierra--you can keep your drives formatted HFS+ if you want.
There will likely come a time when that changes, of course.
And as stated above, just because they
say it won't work when it's changed, it may still work anyway. Maybe for a long time. But they may also (as I found out) make it a nuisance to try to continue doing it the old way, y'know, to
encourage you.
