Non-resident landlords haven't submitted self-assessment tax returns for 9 years. They own property in the UK but are not resident (nor domiciled) here. If they had submitted the returns they wouldn't have paid any tax due to personal allowances.
Do they still need to make a disclosure under the Let Property Campaign even though there is no tax to pay? Or is there some other way to tell HMRC? I have prepared the 2020 returns.