https://www.psacard.com/pop/multi-sp...e-series/49281
Here's another set from the same years that has been graded pretty heavily for a multisport issue. The cardstock is much thinner than a T206, but out of 1619 cards for the 1909 (first editions) and 445 for 1910, highest grade is 8.5. Total of 2064.
8.5: 1
8: 9 (plus some qualifiers)
7.5: 4
7: 46
6.5: 5
6: 69
I graded and own the 8.5 and half the 8s; most came out of a really nicely collated set I bought from Kit Young that looked fine to me before I submitted them. So there are NM-MT examples that have survived all this time, but I agree with your premise that it's likely many of the high grade T206 cards in PSA 8-10 holders have been altered/trimmed over time.