Just curious - Can a person legally transfer the rights of their pdf to another person or to a library to lend out? I don’t know the actually answer but it seems pertinent.
if a company sets themselves up as the only way possible to procure a work and then chooses to stop selling a digital copy, and none of those digital copies which were sold can be transferred individual to individual - then IMO that’s not ethical (regardless of whether you call it censorship). I don’t care whether they continue to sell it or whether the simply update their terms to allow the already sold work to be transferred between individuals (if it isn’t legally possible already).
You're into a very complex set of topics related to IP laws. IANAL, so I cannot really give you a legally useful opinion. OTOH I have some experience as a creator of certain kinds of works... When you sell a physical item there is, in the US, a doctrine, the 'Doctrine of First Sale' which states that the original creator gives up their rights with respect to THAT INSTANCE of what was sold. Thus if you bought a copy of OA from TSR in 1986, you own it. You can sell it, lend it, destroy it, give it away, show it to people, whatever you want, as long as you don't reproduce it. However, it is less clear with digital material. First Sale DOES still apply, but it is a whole lot less clear what constitutes an 'original' when dealing with digital material! Libraries HAVE successfully argued that they can lend material, and they do it all the time.
There is also the question of 'Fair Use', a copyright is not an absolute right. The public has certain rights WRT a work, including the right to reproduce parts of it for educational purposes, criticism, etc. within certain limits (basically as long as it doesn't degrade the value to the rights holder, or deprive them of the benefits of their rights, but this is a super complex area of law that is virtually decided on a case-by-case basis).
So, WotC, as the copyright holder of the work, OA in this case, can stop selling it. They can't do much else. MAYBE they could come after you for 'selling your copy of the PDF' online, but they would probably have to prove there are effectively multiple copies in existence as a result. Libraries can probably lend it out too, that's pretty established.
A FURTHER complexity is that not everything that people got in their hands was 'free and clear'. Especially in modern times businesses have taken to 'licensing things out'. That is they claim they didn't really sell it to you, that they "still own it." This has sometimes worked, and sometimes courts have basically said "If it quacks like a duck, its a duck!" There can also be NDAs and various contractual arrangements that could limit what can be done with media, though they are not generally very applicable to something that is being sold to the public.