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.
The effect would similar to America/Fort_Nelson that switched from PST/PDT to permanent MST on March 8/2015. $ TZ=America/Fort_Nelson date -d 'Mar 7 2015' Sat Mar 7 00:00:00 PST 2015 $ TZ=America/Fort_Nelson date -d 'Mar 9 2015' Mon Mar 9 00:00:00 MST 2015 -chris