If you control more than 25% of the shares or voting rights of a company, have influence over that company, and can appoint directors, then you are a person with significant control (PSC).
Your status as PSC will have been (or should have been) recorded in the company’s record at Companies House.
HMRC is now using this information.
If you are a PSC and have not submitted a SA tax return then expect a letter from HMRC.
There may be a good reason why you do not submit a tax return such as taking nothing from the company or not having any income that needs to be reported in this way, but you will be expected to confirm to HMRC that this is the case.
HMRC may issue a failure to notify penalty if you have not registered for self-assessment when required to do so.
If you are indeed a PSC then you should consider whether you need to pre-empt HMRC and get registered for Self-Assessment.