Unix time is bad and needs replacement, not UTC

Another example is Xinjiang province, where in some towns everything is in local time, in some towns everything is in legal time, in some towns it depends on if you're Han or indigenous, and in some towns it depends on what you're doing (bus timetable vs shop opening hours).

Another example is in a very large but lightly populated part of Australia, where there local community uses a defacto timezone (Australia/Eucla) but the legal timezone is different (Australia/Perth) - it is no use saying that the polls close at 6pm and the general store closes at 6pm and coding them with the same coordinates, because one will refer to a moment 45mins after the other. Other places may have similar variations of opinions where the formal boundary of two timezones isn't really obvious - I have heard that the timezone boundary around Broken Hill is a matter of opinion as much as it is a matter of law.

The other possibility is of course that perhaps in a country like the US where timezone boundaries are relatively unstable over time because they pass through states. It's true that coordinates could be used to deal with it. But it's not obvious that this is such a systematic and regular form of variation that timestamps should use coordinates, especially considering almost any record that relates to time in a place probably already codes the place so an ad hoc migration should generally be possible. (In such a case, I would still like tagging the time with a tzid so you know for a fact whether it is premigration or a postmigration.)

I see a lower-bound case for two: sun/Earth time and universal time. Farmers must be able to express "I will plant seeds at noon on August 29, 2329". They care about how the sun affects Earth. The worker trying to schedule a meeting is the same, because they'll want to guarantee the meeting happens during daylight. As such, I find the obsession over "always use Unix timestamps" a bit weird. Yeah, they're simple, but semantically wonky for some practical things. Everyone else, dealing with durations and time deltas unrelated to geological conditions want TAI.

For the farmer, the problem is the Gregorian calendar is deeply flawed/inaccurate, not that UTC has/had a leap second. Sun and Earth rotations are not aligned, so we cannot expect these to line up. We need year, day of year and time of day. The year is just the "number of solar turns" integer. The first and last day of the year would have to be cut up in unsavoury ways to account for misalignment. The time of day only needs to line up with Earth's rotation, but this means the speed relative TAI will need to shift as Earth is affected/slowed by the universe. (Now, I would suggest making the clock zenith based on something around the poles or leaning of the Earth axis, to end the Greenwich bias: that's pretty damn arbitrary.)

I guess what I'm really saying is that UTC-without-LS won't be UTC; rather, we'd declare that the new universal civil-time timescale is to be UTC-without-LS. You could carry on computing old-fashioned UTC after the transition; but one wouldn't generally use UTC to render civil-time for display.