A tax blog worth following, quoting another blog by the National Taxpayer Advocate, informs us:

"According to the NTA, the IRS sent Notice CP21C informing taxpayers that the IRS was offsetting their Economic Impact Payment (EIP) to satisfy outstanding debt.  Here is the critical language from the notice as recounted in the NTA blog post:

We applied a credit to your 2007 [that is not a typographical error!] tax account due to new legislation. We used (offset) all or part of your economic stimulus payment to pay your federal tax as the law allows … As a result, you don’t owe us any money, nor are you due a refund.

The IRS did not offset their EIP and tax year 2007 has nothing to do with the issue. 

The people receiving this notice appear to be people who did not have a 2018 return on file and who filed their 2019 return too late for the IRS to process their EIP before the end of 2020."

IRS mailed these to 109,000 taxpayers.

https://procedurallytaxing.com/more-trouble-with-notices-and-more-discussion-of-offsets/