I agree with Michael D.
There are four existing zones that are already on permanent UTC-7: America/Dawson_Creek America/Creston America/Fort_Nelson America/Phoenix None of these zones makes use the DST bit. The time zone is displayed year round as MST instead of PDT. I don't think anybody complains about it.
I don't see any compelling reason to treat the two Yukon zones (America/Whitehorse & America/Dawson) any differently than the four existing UTC-7 zones.
Historic timestamps? Historic timestamps and their time zone abbreviations do not change. Nobody is suggesting that they should be changed. e.g. America/Dawson already supports all the historical time zone abbreviations that have been used in Yukon Territory since 1900: PDT, PST, YDDT, YDT, YPT, YST , & YWT. The proposed change would force all dates that occur after March 14/2021 to have an MST time zone abbreviation. All dates that occur before March 14/2021 would continue to be displayed with the correct historical time zone abbreviations. Correction: the change takes place this upcoming weekend (March 8/2020). It is not March 14/2021 as I indicated in my previous e-mail. -chris