The current proposal as far as I understand it is that they will tell a company/service to "voluntaraly" scan the content.
Then an open source project that is forced to comply can just stop development and be forked on the last commit and shrug shoulders.
Which will buy enough time, because legally making those requests will take time and work. I can easily see fork related infrastructure for end users making it really eash or automatic to keep up with that cycle.
But even better would be to develop open source hardware based communication solutions, that don't have the ability to add a backdoor through a software patch. Think like a pager with all the security and privacy bells and whistles.