It’s called “deaccessioning,” the reverse of acquisition. It’s also controversial in many cases, because the job of a museum is to preserve, and selling off pieces of their collections is the reverse of that. But it’s not all one thing, a monolith of bad policy used to shore up a sinking ship torpedoed by rash decisions. Sometimes there are good and healthy and right decisions that lead to the practice.
I’m mostly referencing this Hyperallergic article, and as they point out, some museums are deaccessioning works to diversify their collections beyond the traditional white-male-heavy stacks. Sometimes they’re honing their educational mission. As we patronize and contribute to museums with our time and money, we ought to keep the big picture in mind. What does the overall collection look and feel like? How are they living up to their values? And so forth.