So it's evident that with regard to this problem, Yahoo are acting with gross unprofessionalism.
Furthermore, Yahoo gave no advance warning of their decision to 'disable' generation of app passwords last May, the Account Security page gives no indication that it's been disabled (until you actually click the button and the error appears), and all their Help pages relating to POP/SMTP/IMAP still state that app passwords can be generated, even though it has not been possible for months.
Likewise, I've been working in IT for decades (30+ years), and it's f'ing obvious that after 5 months of not resolving this fault, Yahoo do not consider this to be important, are making no effort to fix it, and don't give a sh*t that their customers are greatly inconvenienced. 'The developers are still working on the issue' sounds like total BS to me.