Wrong. There was full app compat of WP7 apps in WP8 and Win10 Mobile, and for WP8 apps in W10M. The only full backward app compat break was from WM6.5/WP6.5 to WP7.

I'll give you the benefit of the doubt and assume you're thinking of the lack of device OS upgrades: from WP6.5 to WP7, from WP7 to WP8, and from older WP8 devices to W10M. So no forward compat, but absolutely yes to backward compat.