My office recently started using Keynote for a lot of our presentations. Keynote presentations created on the version that is compatible with Yosemite (and nothing prior) cannot be opened on earlier versions.

Even if security updates still come for what...usually 20-30 months with Apple...they still don't care about any other kind of backward compatibility and happily break their own file formats at will.

Just to check, do Apple only provide security updates to the newest OS X?

No. Apple provides security updates for several versions. They most recently patched NTP for Mountain Lion, Mavericks and Yosemite at the same time.

http://support.apple.com/en-us/HT1222

Apple provides security updates back somewhere between 36 and 44 months for the OS. Unfortunately, you won't know until they announce some exploit and don't patch your OS if you've been left behind or not.