This builds on both (1) the requirements and vetting procedures that are already in place for formal visa applications to the USA and (2) the optional questions already on the ESTA, so it's quite feasible and doesn't require new background routines. There's a comment period of 60 days before this goes into effect so it could be stopped, but it's also very possible it will go forward.
If you genuinely have no SM (never had, or all accounts deleted more than five years ago) you answer NONE. (Reddit is specifically included on the "including but not limited to" list of mandatory disclosure platforms, so presumably you should disclose Mumsnet too? ) If you do have accounts, you list each platform and all active handles on that platform. In addition to vetting any accounts listed, the AI checks for any undisclosed SM accounts, presumably using your name and the full set of email addresses and telephone numbers you've provided (you'll be required to submit EVERY ONE you've used in the past five years) and if they find no SM presence at all or if they find no undisclosed accounts and no "bad" content on the disclosed accounts, then presumably there's no issue. If everything doesn't check out, the system spits out your application for review by a carbon-based life form.
The requirement to make all social media accounts public is already an issue as some people - disproportionately women - have legitimate reasons to keep them private. If these policies were properly vetted, they would require applicants to grant access to all accounts to Homeland Security (or its designated agent) not to make them public.
Many people can and do avoid travel to the USA, but a lot of international flight itineraries pass through the USA and since there are no airside connections for international flights (for example, a flight from London Heathrow to Cancun with a change of plane at Miami Int'l) all passengers go through US immigration. If this is implemented and appears to be a perm change, I wonder if we will eventually see more flights from Europe to the Caribbean and Central and South America routing through Canada or Mexico instead?