The angular momentum of the "earth system" is constant [1].
The variation in angular velocity of the solid earth can be ascribed to change in the angular momentum carried by oceans and atmosphere. These changes can be due to change in velocity (winds, currents) or moment of inertia (moisture in atmosphere).
[1] If we ignore friction due to tidal forces from moon and sun (which is valid on these time-scales), the "earth system" is not affected by any non-central forces and so the angular momentum is constant.
But tidal forces are not completely irrelevant to leap seconds.
The length of the atomic SI second was calibrated to match the previous sidereal second, which was based on the length of a year. In practical terms, the sidereal second was based on Newcomb’s Tables of the Sun, which was a model of the solar system developed at the end of the 1800s based on historical astronomical observations https://en.wikipedia.org/wiki/Newcomb%27s_Tables_of_the_Sun
So the length of the second is based on the length of the year 1900, and derived from older astronomical measurements of the rate of movement of bodies in the solar system.
Leap seconds exist because by the 1960s the length of day no longer matched Newcomb’s calculations for 1900: it was longer by 1 or 2 milliseconds.
Is the total mass of all mined minerals and hydrocarbons more than rounding error in the calculation of moment of inertia?
This would assume that mined materials stay at a higher elevation than they were. And that the materials above those items didn't collapse down and/or aren't heavier than the mined mass.
Slowing down of the earth is caused by wind turbines, just as extracting energy from the tides causes the moon to fall towards the earth. When the earth will have completely stopped rotating, and the moon be dangerous close to the earth, that will be yet another shitshow. And they call this "renewable energy"?
[1] If we ignore friction due to tidal forces from moon and sun (which is valid on these time-scales), the "earth system" is not affected by any non-central forces and so the angular momentum is constant.