According to an article published by The Atlantic in late October:
A political scientist at Carroll College, working on behalf of plaintiffs challenging Ohio’s signature-matching law, found that 97 percent of rejected signatures are likely to be authentic—or, for every invalid ballot, 32 valid ones are thrown out.
Even in normal election cycles, signature-matching requirements result in many ballots being rejected. Hundreds of thousands of such ballots were disqualified this way in 2016—almost all, presumably, cast by voters who had done everything right. Rejections disproportionately hit certain demographic groups, including elderly voters, young voters, and voters of color, that are expected to heavily favor Vice President Joe Biden this fall. As voting by mail surges across the country, many elections, including the presidential race, could hinge on a process that one expert recently described to me as “witchcraft.”
The article then goes to discuss in more factual terms how the signature matching standards vary across states and how in some states the guidelines have changed to tilt towards accepting rather than rejecting a questionable signature. (Sometimes such a change was in the context of lawsuits, e.g. in Pennsylvania.)
Are there any statistics or even estimates informed from actual 2020 patterns/samples (not extrapolations using 2016 data) of how many mail-in votes were rejected across the US for failing the signature match test in the 2020 election?
N.B., ABC provided a more detailed account for 2016 and 2018:
Why were so many mail-in ballots tossed out? The biggest reason cited by local election officials is, simply, the lack of a valid signature.
ABC News analysis of EAVS data from 2016 and 2018 revealed that nearly 290,000 mailed ballots were scrapped because the voter did not properly sign the ballot, signed in a way that did not match their registration signature or did not have a required witness validation.
Signature issues account for about 57% of the more than 500,000 mail-in ballots with “identifiable” reasons for rejection in 2016 and 2018, followed by voters missing the deadline to return ballots, which accounted for about 37% of rejections.
I suppose a part of this question is also how many of the initially rejected signatures were "cured" in 2020, because...
Twenty-two states notify voters and give them a chance to cure, or fix, ballot signature defects within a few days of the election, according to the Brennan Center, a nonpartisan law and policy institute. But 28 states -- including the key battlegrounds of Michigan and Wisconsin -- do not.