Yea

Generally, disclosing disabilities when applying is voluntary. So the only problem is if they up front said they didn't have a disability. It is neither unethical nor illegal not to mention it during the application process.

It absolutely is unethical if it affects your ability to fulfil the agreed terms to that extent.

Would it not be unethical for the employer to advise you on your first day that, actually, the company is struggling and can only pay you 60% of the agreed wage?

In most places yes, you aren’t allowed to even ask these questions during an interview since it can introduce a bias.

There is a major risk to employers pretty much any western country when it comes to these sort of discrimination claims.

You can't ask about disabilities, but you can ask about whether the applicant can fulfill certain expectations that could eliminate candidates with certain disabilities. For instance, "This job requires you to be onsite for 9AM to 6PM every day, are you able to do that?" or "This job requires you to operate heavy machinery and lift 50lbs multiple times per day, are you able to do that?"

That’s true but it’s also easier for certain jobs than others.

Also depending on the jurisdiction and circumstances they might say that they honestly believed that they would be capable of it but realized that they aren’t and you might be obligated to accommodate them to a certain degree.