Version 16

V16 plugins keep backwards compatibility is intact, however note that there is a new optimized storage system using a new format.

If you save projects with the new version, old versions won't be able to read the settings.
If you'd need to downgrade for any reason, first go to "Settings / Compatibility settings" and enable "Storage compatibility mode for V15", then save the settings.
Also note that V16 no longer provides 32-bit plugins on Windows. You should be able to run V15 32-bit along with V16 though. Simply do not uninstall V15 and do not perform plugin cleanup.