This essentially means being less anal about ensuring the earth has the exact same clock time at the exact same earth-sun-relative rotational position. Turns out the earth spins up and slows down semi-randomly with a magnitude big enough to require positive or negative leap seconds every few years, and it's just not worth the effort. Leap minutes might only be needed around every 100 years, but that would still be annoying, so they dropped all the way to leap hours, probably not needed for thousands of years. As long as the sun rises in the morning and sets in the afternoon, that saves on a lot of frequent minor adjustments. Timezones are already an hour wide anyways, and the sun isn't in the same spot when you're on the western vs eastern edges.

Sure, you could also run a relativistic simulation of the solar system, galaxy, and cosmos to establish a more monotonic clock and calendar independent of imperfect orbital parameters, but that's a bit much for discerning whether or not it's lunchtime.

Several hundred years at least (see my sibling comment). And in all likelihood this will be handled by shifting the time zones by an hour, or possibly by omitting one DST adjustment in spring.

In the very far future, leap seconds would become inadequate anyway, as you’d have to insert them every month, then every week, then every day, due to the deceleration of Earth’s rotation.

in 100 years the Sun will be inside the Dyson sphere, so there will be no need for further solar time adjustment