Related to this, I've observed an interesting oddity this year that is increasing the number of self-rated players.
There are a fair number of players who had 2017 year-end ratings, didn't play enough in 2018 or 2019 to get a year-end rating, but did play enough in 2020 to get a year-end rating, but obviously didn't get one since on year-end ratings were published. These 2017 ratings are now 3 years old and expired, and these players must self-rate again for 2021. Similarly, there are seniors who have a 2-year expiration and had a 2018 year-end rating and played enough in 2020 to get a new year-end rating, but now have to self-rate again.
So this also inflates the number of self-rates in 2021.
The bottom line is the USTA elected to treat 2020/2021 as one rating period for some things and accept the consequences of that, but then compound that by having 2017 and 2018 (senior) ratings expire even though there was no 2020 rating period.