Hello,
Arthur and I have been emailing affected exit operators for a while now and many operators fixed their exits but some minor number or exits remain broken (even some that are running good tor versions).
I believe it is reasonable to assign the BadExit flag to exits failing 100% of DNS resolution attempts and have a CW > 200 (or any other value you like, I just would not bother about exits with a CW of 20) starting with 2018-01-15, if we did send them an email about the problem (if they have a contactInfo set).
These are 4 exits failing 100% of DNS resolution attempts based on Arthur's scan data from 2018-01-16 05:10:00 UTC
+------------------------------------------+-------------+-------+----------------------------------------------------+ | fingerprint | tor_version | CW | contact | +------------------------------------------+-------------+-------+----------------------------------------------------+ | 6F6586F1631CAC1115B9561656E7664D386B7F4B | 0.3.1.9 | 404 | Ins0mnia Shells <orwell1984.info AT gmail dot com> | | 34D3AF8DC4CF68BDEA354D21C24EFB2EAFD7191F | 0.3.1.7 | 2040 | tor@krosnov.org | | 92E3764D5485DC4AC01178271FB5A8A2D90DA9FF | 0.3.1.9 | 16700 | Random Person tor-admin@portaltodark.world | | 60A7451D512B647C0D05A2063CA7F329311347B5 | 0.2.5.16 | 21200 | NULL | +------------------------------------------+-------------+-------+----------------------------------------------------+