If the IRS informed a client that their Stimulus Payment (EIP 1-3) was deposited to their account on such-a-such date, and the client contacts us after that date stating the deposit to their account is missing...is that subject to Reg E Error Res investigation, timing, and notices?

Yes - a missing deposit is a Regulation E error - however you are limited to investigating your own records to see if you received it. Do they have a record to which deposit account they instructed them to deposit it to?

If the IRS informed them, it should also inform them of the date the payment should have been credited and the last four digits of the account number. That should take you right to the correct places in your account records and ACH files.