They are setting boundaries. They are cutting back on their commitment and engagement with work because they see that work is not the most important thing in life. They make decisions based on their mental health instead of the company’s bottom line.
No one is leaving their job in this situation. No one is not doing their work. They are simply not taking on extra work and commitments that they aren’t getting paid for.
Our society’s relationship with work is so skewed that the word we have chosen for this is “quitting”. There’s something profoundly sad about that.