Will they be identified by Social Security, Railroad, or government pensions.

First, why is anyone worried about unfiled 2018 returns?  Filing a 2019 return would achieve the same result -- although the unanswered question is the cutoff date for IRS to determine who gets paid.  

The 2019 can be e-filed.  I keep reading complaints about e-filing the 2018.  Problem is, if there is no refund, the bank account information will probably not process for either year.  

Second, what is a "government pension"?  I assume you mean federal government.  There are military and civilian.  There are also nontaxable VA benefits -- I'm fairly certain those will be included.  The problem with many of these will be that a lot of $1,200 payments will be made to households where a married couple is eligible for $2,400.  

Rather than beating the bushes for returns that might not have to be filed, I would ask clients if they know someone who is a nonfiler and deserves help.