Right now, when a backup file was created in a site newer than the site where it's restored a warning is produced:

This backup file has been created with Moodle 3.2dev (Build: 20160523) (2016052300) and it's newer than your currently installed Moodle 3.0.4+ (Build: 20160520) (2015111604.08). This could cause some inconsistencies because backwards compatibility of backup files cannot be guaranteed.

And still, we always allow the user to continue with the process (knowing the risks).

And we have had not many problems with that till now.

But around 2.9/3.0 we started including various version-based decisions (gradebook mainly), calls to functions and friends that may lead to broken restores way often.

So this is about to reconsider if that should be a warning or we should forbid it more actively (because possibility of breakage is really higher).

There are 2 possibilities here:

1) The aggressive: We forbid ANY restore into older site completely. Simple. Just transform the warning into an error and done.
2) The less-agressive: We forbid (error) restore operations to sites running OLDER BRANCHES, but keep the warning when restoring on same branch site. This may help some cases where teacher's site is more updated than production site, but within the same major version.

So this is it, ciao

PS: I'd consider this a dataloss and recommend whatever solution is agreed to be backported to security branches.