Hi there, Signal dev here. The new backup format is indeed cross-platform. I've successfully restored backups on an iPhone, we're just stabilizing things :)
If you're curious, the reason that Android's current local backups aren't cross platform is because it was made a long time ago, and it's literally a dump of all the sqlite statements that can be used to recreate Android's sqlite database (encrypted with a strong, random, local key). So not the most portable!
But this new thing is all cross-platform, and in the near future we'll even be making our local backups cross-platform.
If you're curious, the reason that Android's current local backups aren't cross platform is because it was made a long time ago, and it's literally a dump of all the sqlite statements that can be used to recreate Android's sqlite database (encrypted with a strong, random, local key). So not the most portable!
But this new thing is all cross-platform, and in the near future we'll even be making our local backups cross-platform.