Bug 997114 removed EUC-TW from m-c. comm-central should either import the code or remove the traces in charsetalias.properties, etc. I recommend the latter, since it appears that this encoding has always had an x- label and has never been registered at the IANA and, therefore, people probably haven't cared about using this encoding for email. (The Traditional Chinese localization defaults to UTF-8 for outgoing and Big5 for incoming.) There is no Bugzilla record for the motivation for adding this encoding in the first place, though it might have had something to do with system encodings on proprietary Unix systems (AIX and Solaris in particular).
Created attachment 8526008 [details] [diff] [review] bug1026946_react_to_rem_EUC-TW.patch Remove all traces of it.