Having lived through many periods when WEB did not buy back stock and probably should have, I don't think that he would repurchase even one share at less than a 10% discount to his own calculation of IV...

There seem to be quite a few factors going into when the buybacks happen. It's not just the prevailing price or price/IV.
They seem to go on at relatively high valuations sometimes, and seem to stop at relatively low valuations at other times.
I think other factors are other [potential] short term uses for the money, and whether there is a material bit of undisclosed news (or just statements) imminent.
Not necessarily for regulatory reasons, but I suspect that things stop for a while before statements are out if the statements have any sort of surprise.

Speaking of buybacks, the overall cumulative effect of the buybacks is starting to show a bit.
In terms of economic interest, 12.02% of shares have been retired since the temporary peak share count five years ago. Due to conversions, the A-share count is down by 22.93% and the B-share count is down by 2.95%.
The count is down 12.34% from the all time peak which I think was end 2011.

I certainly don't read anything into it when buybacks stop.
But the highest valuation level at which they have taken place is worth noting.

Jim