Wait what? Is there a list somewhere? AFAIK interactive brokers (which doesn't engage in PFOF) also shut down trading.
As to the other point, I belive the issue was in settlement, not clearing, ie with DTCC collateral reqs.
A bunch of others had a problem when Apex Holdings told them to restrict trading (or something along those lines).
[1]: https://investorjunkie.com/stock-brokers/broker-clearing-fir...
Also, Robin Hood - the irony. They are clearly for the rich against the poor. More like the sheriff of nottingham!
Maybe because there's already a high barrier to trading US stocks (or stocks in general), so there wasn't enough people piling on to strain the brokerage's balance sheets?
Citadel is an execution broker. Their job is to match buyers and sellers.
Apex (in these conversations) is a clearing broker. Robinhood is its own clearing broker. Clearing brokers are responsible for ensuring that money and stock actually changes hands (this takes 2 days, but all of these firms work together to create the illusion, using credit, that it's instant; those credit arrangements are why brokerages post collateral).
Clearing brokers are members of clearinghouses. The relevant one here is (I think) NSCC, which is owned by DTCC. Policy set at DTCC determines how much collateral needs to be posted to cover any particular set of trades.
DTCC drastically ratcheted up the amount of collateral required to cover trades in meme stocks, which had the effect of 10x'ing the amount of cash Robinhood was required to post to insure that it would not go out of business before its current set of in-flight trades cleared. It made the same requirement of Apex, which passed restrictions down to its customers. These companies are contractually required to make good on collateral requirements, so there isn't much choice involved.