I hope that Maemo Leste doesn't go to forever update hell (if you upgrade new version of OS you have to install new version of all software or if you want install new version of software you have to install new version of OS). That is why I don't like iOS/Android/modern macOS (old Mac OS didn't have that problem)/modern Windows/modern Linux. Update stream model sucks!

Where is downcompability like with Symbian/DOS/TOS/System(Mac OS)/AmigaOS/Windows(up to 98SE)
If software doesn't need new features of OS, you could run it in older version of OS or you could run old software in newer OS. I think one best example is to run Mac System 1 software (from 1985) in Mac OS X 10.4.11PPC (from 2007). And yes you could do opposite too, if you didn't use developing tools based gcc.

Sorry this busting, but I hope we(I hope that I code something to Leste later) take versioning seriously and make programming documentation easy to follow what calls works in what version.