In a post on this list on Nov 24 last, Garret Wollman said regarding FreeBSD's c.1994 change (now reversed) to not install "backward":
I chose not to install the "backward" zones in the hope that this would more securely deprecate the then-recently-obsoleted old-style names like "US/Eastern", and then maybe they could be completely removed from the distribution if other downstream consumers (only a few operating systems at that point, no language runtimes) would do likewise.
Unfortunately, these ancient compatibility links never got deleted, so "backward" now contains a mix of ancient and more recent compatibility links, and users are still using them (or being told to use them by documentation that should have been updated decades ago).
The obvious way to resolve this would be to split "backward" into two: one file containing aliases that were once regarded as canonical names (or at least names consistent with current naming theory) but were then renamed (e.g. the recent example of Godthab -> Nuuk), the other containing all the ancient stuff that never conformed to the current style (such as "W-SU", "Poland", "GB-Eire", "US/Eastern" etc.). One way to do this might be to set a cutoff date, for which the 2000a data release seems like a good choice, and retain only the zones changed since then in "backward", moving the remainder to a new "deprecated" category. Another way would be to reserve "backward" for zones that have appeared in zone.tab at any time, and move the rest to "deprecated". It turns out that these two methods give almost exactly the same results, with only two exceptions: America/Ensenada was in "backward" before 2000 going by the SCM history, but existed in zone.tab until 2010; and Asia/Ulan_Bator was added to "backward" and removed from zone.tab in the 2000a release. This suggests that the zone.tab criterion is a slightly better one. (One further exception should be made: the link from Etc/UTC to UTC is in widespread use and should probably be moved to "etcetera" alongside the similar "GMT" link. Otherwise it should stay in "backward". This change already exists in FreeBSD, where all "GMT" defaults have been changed to "UTC" at some point.) Split this way, the list of links in "backward" would become (posted in plain text here for readability, I can submit in patch form if needed) as follows: Link Africa/Nairobi Africa/Asmera Link Africa/Abidjan Africa/Timbuktu Link America/Argentina/Catamarca America/Argentina/ComodRivadavia Link America/Argentina/Buenos_Aires America/Buenos_Aires Link America/Argentina/Catamarca America/Catamarca Link America/Atikokan America/Coral_Harbour Link America/Argentina/Cordoba America/Cordoba Link America/Tijuana America/Ensenada Link America/Nuuk America/Godthab Link America/Indiana/Indianapolis America/Indianapolis Link America/Argentina/Jujuy America/Jujuy Link America/Kentucky/Louisville America/Louisville Link America/Argentina/Mendoza America/Mendoza Link America/Toronto America/Montreal Link America/Rio_Branco America/Porto_Acre Link America/Argentina/Cordoba America/Rosario Link America/Tijuana America/Santa_Isabel Link America/Denver America/Shiprock Link Pacific/Auckland Antarctica/South_Pole Link Asia/Ashgabat Asia/Ashkhabad Link Asia/Kolkata Asia/Calcutta Link Asia/Shanghai Asia/Chongqing Link Asia/Shanghai Asia/Chungking Link Asia/Dhaka Asia/Dacca Link Asia/Shanghai Asia/Harbin Link Asia/Urumqi Asia/Kashgar Link Asia/Kathmandu Asia/Katmandu Link Asia/Macau Asia/Macao Link Asia/Yangon Asia/Rangoon Link Asia/Ho_Chi_Minh Asia/Saigon Link Asia/Thimphu Asia/Thimbu Link Asia/Makassar Asia/Ujung_Pandang Link Asia/Ulaanbaatar Asia/Ulan_Bator Link Atlantic/Faroe Atlantic/Faeroe Link Europe/Oslo Atlantic/Jan_Mayen Link Etc/UTC Etc/UCT Link Europe/London Europe/Belfast Link Europe/Chisinau Europe/Tiraspol Link Pacific/Honolulu Pacific/Johnston Link Pacific/Pohnpei Pacific/Ponape Link Pacific/Chuuk Pacific/Truk Link Pacific/Chuuk Pacific/Yap Link Etc/UTC UTC "deprecated" would contain these links: Link America/Adak America/Atka Link America/Indiana/Indianapolis America/Fort_Wayne Link America/Indiana/Knox America/Knox_IN Link America/Port_of_Spain America/Virgin Link Asia/Jerusalem Asia/Tel_Aviv Link Australia/Sydney Australia/ACT Link Australia/Sydney Australia/Canberra Link Australia/Lord_Howe Australia/LHI Link Australia/Sydney Australia/NSW Link Australia/Darwin Australia/North Link Australia/Brisbane Australia/Queensland Link Australia/Adelaide Australia/South Link Australia/Hobart Australia/Tasmania Link Australia/Melbourne Australia/Victoria Link Australia/Perth Australia/West Link Australia/Broken_Hill Australia/Yancowinna Link America/Rio_Branco Brazil/Acre Link America/Noronha Brazil/DeNoronha Link America/Sao_Paulo Brazil/East Link America/Manaus Brazil/West Link America/Halifax Canada/Atlantic Link America/Winnipeg Canada/Central Link America/Toronto Canada/Eastern Link America/Edmonton Canada/Mountain Link America/St_Johns Canada/Newfoundland Link America/Vancouver Canada/Pacific Link America/Regina Canada/Saskatchewan Link America/Whitehorse Canada/Yukon Link America/Santiago Chile/Continental Link Pacific/Easter Chile/EasterIsland Link America/Havana Cuba Link Africa/Cairo Egypt Link Europe/Dublin Eire Link Europe/London GB Link Europe/London GB-Eire Link Etc/GMT GMT+0 Link Etc/GMT GMT-0 Link Etc/GMT GMT0 Link Etc/GMT Greenwich Link Asia/Hong_Kong Hongkong Link Atlantic/Reykjavik Iceland Link Asia/Tehran Iran Link Asia/Jerusalem Israel Link America/Jamaica Jamaica Link Asia/Tokyo Japan Link Pacific/Kwajalein Kwajalein Link Africa/Tripoli Libya Link America/Tijuana Mexico/BajaNorte Link America/Mazatlan Mexico/BajaSur Link America/Mexico_City Mexico/General Link Pacific/Auckland NZ Link Pacific/Chatham NZ-CHAT Link America/Denver Navajo Link Asia/Shanghai PRC Link Pacific/Pago_Pago Pacific/Samoa Link Europe/Warsaw Poland Link Europe/Lisbon Portugal Link Asia/Taipei ROC Link Asia/Seoul ROK Link Asia/Singapore Singapore Link Europe/Istanbul Turkey Link Etc/UTC UCT Link America/Anchorage US/Alaska Link America/Adak US/Aleutian Link America/Phoenix US/Arizona Link America/Chicago US/Central Link America/Indiana/Indianapolis US/East-Indiana Link America/New_York US/Eastern Link Pacific/Honolulu US/Hawaii Link America/Indiana/Knox US/Indiana-Starke Link America/Detroit US/Michigan Link America/Denver US/Mountain Link America/Los_Angeles US/Pacific Link Pacific/Pago_Pago US/Samoa Link Etc/UTC Universal Link Europe/Moscow W-SU Link Etc/UTC Zulu -- Andrew.