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The main people who need to upgrade are people who operate a bridge relay -- so their bridge address can resume being given out to censored users, and so their stats can resume being included in the metrics pages.
Changes in version 0.3.3.9 - 2018-07-13 Tor 0.3.3.9 moves to a new bridge authority, meaning people running bridge relays should upgrade.
o Directory authority changes: - The "Bifroest" bridge authority has been retired; the new bridge authority is "Serge", and it is operated by George from the TorBSD project. Closes ticket 26771.
We also put out new stable releases for 0.2.9.16 and 0.3.2.11, and those releases include other backported fixes: https://gitweb.torproject.org/tor.git/tree/ReleaseNotes?h=tor-0.2.9.16 https://gitweb.torproject.org/tor.git/tree/ReleaseNotes?h=tor-0.3.2.11
--Roger