Here's 2008 before election day:
It’s Tuesday, November 4th, 2008, Election Day in America. The last polls have straggled in, and show little sign of mercy for John McCain. Barack Obama appears…
fivethirtyeight.com
Looks like they didn't assign probabilities back then like they do now. But it would have been a very high probability. As Nate Silver explained recently, for a given polling lead, the chances of winning improve the closer you get to election day (since the time for the underdog to close the gap is depleting).
92% in 2012 (despite a close margin in national polls):
Mitt Romney has always had difficulty drawing a winning Electoral College hand. Even during his best period of polling, in the week or two after the first presi…
fivethirtyeight.com
Keep in mind that the question isn't really if Biden wins a free and fair election (he has most pathways to victory). The question is whether any states will significantly prevent a free and fair election, and if enough states do so that could create chaos and force SCOTUS to intervene in vote counting. I believe one of Silver's recent articles discussed that he has no way of modeling this uncertainty.