I don't know. You can read through the whole changelog to find out what those features are https://gitweb.torproject.org/tor.git?a=blob_plain;hb=HEAD;f=ChangeLog Or get on their IRC channel and ask them. I doubt it would be a security threat. More likely that a bunch of shit would just be broken for Tor 0.2.1. They blacklisted that Tor version about a year ago. You can see the downtick in relays around August/September 2012: https://metrics.torproject.org/network.html?graph=networksize&start=2012-01-01&end=2013-09-09#networksize I remember seeing that and wondering why the number of relays dropped, and then saw arma mention somewhere, maybe on the mailing list, that they had blacklisted Tor 0.2.1. Another thing is, notice in the changelog that they switched from calling it alpha to release candidate between 0.2.4.14 and 0.2.4.15. That probably means that major bugs, like stuff that causes crashes, have been fixed.