Recently a performance issue was reported related to the removal of nofollow from signatures.
This hasn't been reported before, so it may be related to another add-on or it may be some other underlying issue not related to any add-on.
But, as a precaution, this version removes that feature. This means in terms of signatures the behaviour reverts to the default which is all signatures will be nofollow.
On balance, as the default in XenForo is for signatures to be hidden from guests, having anything other than nofollow is pretty pointless.
You can download version 2.2.0 if you decide the signature functionality is crucial or the performance issue doesn't affect you.