Failback to Original Location
You can fail back to an existing VM in original location from the target host of any version: 2008 R2-2019.
Failback to New Location
If you fail back to an entirely new location or to a new VM in original location, the version of the target host must be equal to or later than the version of the source host:
Target 2008 R2
Target 2012-2019 (the Target OS version must be higher than Source OS version)
Source 2008 R2
Mind the following exception: you can fail back from the source 2016 host to the target 2012 R2 host only if a VM replica configuration is lower than 8.0.