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Firstly, educational use is protected under the first fair use consideration of copyright law. You aren't charging for access to the resource when sharing it, so it's arguably nonprofit; and you're sharing it for educational purpose. So far so good.

Firstly, free educational use can be protected under the first fair use consideration of copyright law. You aren't charging for access to the resource when sharing it, so it's arguably nonprofit; and you're sharing it for educational purpose. So far so good.

If definitely violating copyright law, charging money for copies or something, then it's almost-definitely illegal.

If you are confident that you are definitely violating copyright law, perhaps because you are charging money for copies without permission or something, then it's almost-definitely illegal.

What you're asking, instead, is whether such behavior would be a violation of ethics, which I interpret to mean professional ethics. That is, what an idealistically-respectable member of your profession would do.

Typically, unless there are powerful conflicts between laws and ethics, professional ethics require one to act within the constraints of the law. Violating copyright to save a few bucks wouldn't typically be considered a powerful conflict. So, if you're confident it'd be illegal, then it'd be unethical, too.

Some professors appear to navigate this maze of morality, legality and ethics by giving their students a warning with a wink and a nod, like "The preprint copy at [link] may have been removed. If so, please do not download it from the wayback machine, nor the current edition from sites such as OpenStax, BookBoon, vk, LibreTexts, or similar".

What you're asking, instead, is whether such behavior would be a violation of ethics, which I interpret to mean professional ethics. That is, the explicit standards of behavior for your profession.

Typically, unless there are powerful conflicts between laws and ethics, professional ethics require one to act within the constraints of the law. Violating copyright to save a few bucks wouldn't typically be considered a powerful conflict: no lives are lost if you do not share that preprint. So, if you're confident it'd be illegal, then it'd be unethical, too.

Some professors appear to navigate this maze of morality, legality and ethics by giving their students a warning not to obtain the book illicitly with a wink and a nod, as you see in the comments to the OP here suggesting using the Wayback Machine, or in this widely-spread tweet by a University of Ontario professor recommending that students avoid a list of free textbook sites.