Glad to find this only 1 day old post, because I faced the same problem today. Last drag 'n drop deploy was on May 23. I did not add any files but only updated existing ones.
Using the CLI is not an option, because it’s a Blazor app and Node.js is not installed as instructed on the “Get started with Netlify CLI” site.
Drag 'n drop deploy worked well on a Hello World test site.
Please let me/us know, when drag 'n drop deploy is possible again. I’d like to deploy latest on Friday…
Many thanks!
BR
Philipp