This is totally not true. Usually they make jobs redundant not people. If there's a pool of people doing the same job and that headcount is reduced then it will often be the lowest performers that go however some places have done LIFO or cut the most expensive.
However if you're doing layoffs and you reduce your frontend team the it's likely low performers from the backend team get to stick around.
The one exception might have been when the entire browser division was dropped back in Netscape - everyone was chopped there - but I can't say with certainty whether low-performing Server Division people were impacted (though IT and HR positions were chopped). So - fair, when a division or operating group is cut wholesale, low-performers in other divisions might not be dropped - but knowing the mindset of management - they really like to take advantage of a layoff as a "get out of jail free" card to let someone go. Much less stress, and way, way less paperwork.
If lucky and done right, performance will (inversely) correlate with probability of layoff.
That's interesting, but how would you know? suppose you're in a protected class, and suspect some form of discrimination. How would you fight it?
The EEOC investigates workplace discrimination, so if you suspect it, it would help to file a complaint with them. They can gather evidence to determine if discrimination took/takes placr and hold the employer accountable.