This is the answer from support.
The short answer is it depends on the application 
In general, as long as your generated application (APK) doesn’t have any .so libraries (this includes those from plugins), then you should already be 64-bit compliant.
The potential pain point here is with 3rd party plugins that haven’t been updated to get rid of any .so libraries that are generated or used as part of the plugin setup in your generated APK. I can’t speak for any plugins outside of the Ionic ones, but we will definitely do our best to ensure our plugins are 64-bit compliant by that date (if not already).
I would suggest checking your application and the plugins it uses now to determine whether or not you’ll need to make any updates. Appflow will continue to generate APKs and keep up with standard practices, but we don’t have control over third party libraries so you’ll need to look into that
Hope that helps!