https://fred.stlouisfed.org/series/RRPONTSYD
This is clearly the highest level of reverse repo since the program was introduced, by a wide margin.
There are three factors behind this:
1. The Treasury has temporarily backed off issuance of short term debt as it drains down an overflowing General Account.
https://www.reuters.com/article/us-usa-treasury-liquidity-ex...
2. Banks have been inundated with cash resulting from federal government transfers (stimmy checks, paycheck protection program giveaways, etc.).
3. Banks can't simply accept cash from depositors and be done with it. They need to convert that cash into an asset of some kind. And right now, the asset of choice is short term treasuries (exactly the thing in short supply).
Reverse repo is when the Fed loans treasuries to banks, typically at very low rate and no more than one day. This avoids the need for the Fed to sell its short term treasuries on the open market.
By running reverse repo, the Fed can prevent short term interest rates from falling below zero (they have briefly broken this level in recent months). Such an occurrence would send a very unexpected signal to markets and could result in panic as investors see the value of money market funds shrink for the first time ever.
Of course, you might ask why on earth the Fed doesn't just sell the treasuries it picked up by performing all that quantitative easing over the last 18 months or so. This is what's known as "quantitative tightening." If you want to see markets really freak out, watch what happens if the Fed were to suddenly announce a massive quantitative tightening program.
Whether or not any of this matters is not clear. The Fed still has some tricks up its sleeve to keep the train rolling. And once the TGA is spent down, that could open the path to much more short term debt issuance. Of course, if demand grows faster than supply at that point, things could get crazy.