How many people use CC 2015 or older?

Not really an answerable question, kovart. I'll mark it as such.

Thanks,
Kevin

Are there any reasons to optimize the extension to older version?

Depends on who you are aiming for. Many hobbyists still use CS6 or earlier as do many professionals who couldn't be bothered being forced into Creative Cloud or in addition to it as a fallback option for older projects or just in case. The spectrum of variations on this is endless, including people clinging on to older CC version for dear life because of too many bugs in recent outings. Szalam's statement of AE getting better with every new version is just plain wrong and slightly delusional. That said, if you are a developer, you can only do so much. Since Adobe in their infinite wisdom *sarcasm mode here* decided to screw around with too many things even for custom panels, you can only ever target a handful of versions before things break (unless you really care to maintain 10 different versions). Personally, though, for a commercial project I'd only support latests versions. This train is moving too fast and staying up to date and competitive in that department might be more important than satisfying someoen using an older version who won't be buying new plug-ins and other stuff, anyway.

Mylenium

Sorry to be a wet blanket, but I run a copy of AE CS 5.5 on as legacy machine at home.  It's all mine -- no rental, y'know -- and it's just fine for my volunteer work.

With each release, as AE gets more stable (and faster), fewer people are using older versions. I would expect the release calling itself CC 2018 (even if Adobe is calling it "The October 2017 release of After Effects CC") will win even more people over from CC 2014. Anybody using CC 2015 is either a pirate or silly (or stuck that way due to weird government issues...) as CC 2015 was really buggy. The only old version worth holding on to is CC 2014, but as the new version of AE just added control for (and of) mask paths by expression and other recent releases of AE have added other must-have features, the number of folks holding onto CC 2014 is shrinking.

Flow is the extension I use the most. The new version of Flow doesn't support anything older than CC 2015, if that helps.

I wouldn't trust a poll in a forum like this to be a good indicator of overall version usage either. Some companies like to hold on to older versions for a while. This has always been the way. But a lot of freelancers and such like to be on the very latest. It's a mixed bag.

What extension are you making? I mean, what task does it do?