I'm waiting to hear back from client as to exact vesting date...but ordinary income was recognized at grant date and tax was paid and 83b filed (2016).

But you are saying, if they vested in 2021, right before they were sold, it wouldn't be a (S/T) capital gain?

It is Restricted Stock (not RSU). Reading the Investopedia article, I believe confirms my understanding that this is not ordinary income (even if vested in 2021) but is a capital gain.

Right? It says right there under the 83b Election section, "capital gains treatment still applies."