Yes , as per Notification 78/2020, the taxpayers having AG turnover less then 5 crore is required to mention 4 digit HSN & more the 5 crore its 6 digits.
Yes check has been put on HSN summary table no.12 of GSTR 1, now you cannot file your GSTR1 without HSN or as per above mention.( it gives an error message while filing)