"IRS still be processing 1099s from 2021"

What sort of processing are you expecting them to do?

What type of 1099 was it?

A client with a 1099-NEC or Misc that is from business operations would already have reported everything, and it is not dependent on if someone who paid them issued a 1099-NEC or -Misc for that relationship, since their business should be reported in full and not based only on what is revealed to the IRS.

A client that didn't include 1099-Int or -B or -Div activity would do an amendment for what got overlooked.

A 1099-K is another animal entirely, and depends on what it was for.

There is not always matching by the IRS. They might notice something filed on your client's behalf was not apparently included on the tax return, but maybe not. I had a client that would double-report me on 1099-Misc/NEC, and I told him every year that wasn't how the form was used, and not to worry about it, because my business data was larger than him X2.