What is currently communicated by big Dan: stack add-on developers must actively do “something” to make them compatible, eg. with a converter transpile them into a Elements version.
Of course this “conversion” might be able to be done by someone else, not the actual stack add-on developer, but a 3rd party.
Encrypting the stack add-on by the developers should help.
As all this is not available by now, these are only assumptions. Maybe RW9 will never be released 😝