One of my favorite bits of nonsense in the whole web of nonsense that makes up the discussion around "the public trust" is that when museums (which are, for the most part, private actors who happen to get some tax benefits) go to sell some work, we hear endlessly about how problematic that is because the work is (in some unspecified way) held in the public trust ... but when work is sold by, you know, the public, somehow the public trust doesn't enter into the discussion.
Latest case in point: the Illinois Metropolitan Pier and Exposition Authority is selling a Kerry James Marshall painting that it purchased for $25,000 in 1997, "with public money raised through project-expansion bonds," at Sotheby's next month for an estimated $8-12 million. And of course, not a peep from the Deaccession Police.
So to review: works held by an Illinois municipal authority, purchased with public money: not held in the public trust. Works held by, say, the Art Institute of Chicago, purchased with money they raised from donors, so extremely held in the public trust.
Now, if there were an Association of Metropolitan Pier and Exposition Authority Directors and they happened to have adopted a "Code of Ethics" on the subject, then we can be sure the Deaccession Police (aka Random Code of Ethics Enforcers) would be all over it. But without that Code of Ethics, this work, though held by the public, is obviously not held in the public trust.