I use Mail (app) for Windows 10 (that is installed on my PC) for my Yahoo mail, Gmail, and ISP email. My Yahoo mail is working OK. I have Windows 10 1903.
I just deleted some Yahoo emails with BOTH the Windows 10 Mail app - my Yahoo account AND from my Yahoo Mail webpage (using Edge).
Are you using the W10 Mail app or Yahoo webpage (or which browser) or ???. << I'd login by a different method, and try to delete.
Yahoo mail did have some recent problems; but mine seems to be currently working.
I only use Yahoo mail for joining forums, etc. Yahoo data breach - 2016