Time spent on Wi-fi fell 3% for both Verizon and AT&T customers on average in that period, reaching historic lows of 49% and 51%, respectively. T-Mobile is now on 41%, or a two percentage points drop, but Sprint didn't witness any changes (cue the coverage jokes here).
As you deduced from our unlimited data plans comparison, the market forces have become the great equalizer, and all big carriers now sport plans that are very close to each other when it comes to pricing, though T-Mobile is way ahead when it comes to throttling caps.
That trend is reflected in Wi-fi usage, too. A year ago, the gap between time on Wi-fi for T-Mobile and Sprint, on one hand, and AT&T or Verizon, on the other, was larger, but now that they all offer competitive unlimited plans, that chasm is shrinking. T-Mobile customers are still the least likely to be on Wi-fi among the big four US carriers, though.