"My uncle, had income but was below the filing threshold in 2014-2018. never filed. never got a notice. "
Indeed, I have had clients who were on Social Security (and nothing else) who filed tax returns because they had filed every year for their entire lives so assumed that they were supposed to no matter what...until finally the IRS wrote them a nice letter saying that they no longer needed to file unless they have some other source of income (like taking money from an IRA).
As HACKITOFF says, the IRS won't bother you about not filing if their own records (copies of SSA-1099, W-2, 1099s, etc) don't show that you have sufficient taxable income to be required to file.
