Well, it will still be possible to make broken branches with unfixed clients via SFTP, but those can always be retrieved via SFTP (because in that case the client is able to find the missing data itself, by looking at the fallback repository, whereas the server is unable to use fallbacks).
I suppose pre-1.13 clients will also continue to push up incomplete stacked branches too, because they don't use the RPC that will cause the "please send more keys" response from the server. Hopefully there won't be many of those for long, because 1.13 is much faster at network operations.
Well, it will still be possible to make broken branches with unfixed clients via SFTP, but those can always be retrieved via SFTP (because in that case the client is able to find the missing data itself, by looking at the fallback repository, whereas the server is unable to use fallbacks).
I suppose pre-1.13 clients will also continue to push up incomplete stacked branches too, because they don't use the RPC that will cause the "please send more keys" response from the server. Hopefully there won't be many of those for long, because 1.13 is much faster at network operations.