1.8.9 is still an active version and not everybody is porting to 1.9 yet. I heard those two versions aren't very different code wise, will it be possible to backport newer versions, at least for some months? (until there's a stable version)

No, not worth the effort. I did plan on it but I haven't got the time

Then it's not better to continue developing for 1.8.9 which is the "active" modded version instead of 1.9?

1.9 is the latest version, and if someone doesn't want to update, that's their problem, not ours. They can use outdated and unsupported software, but we won't.

1.8.9 is not the "active" version anyway's where is forge...

1.9 has been out for a month or two, and has 150 mods. 1.8 has been out for a year and a half, and there is 300 mods. 1.8 is dead. move on.

No, 1.9 is we're we are at.

Yeah, forge 1.8 has more than a year and it's starting to get mods now, so i don't think so...
Anyway, it's just a suggestion, good luck with the project.