Joplin web app cannot sync with joplin server anymore?

You could build a release build of the web app as per the instructions here Building the applications | Joplin and then host it as a static web app. Alternatively, you can use this method to host on GitHub pages (which is free) which will allow easy deployment of updates:

This will deploy the latest available version of the web app whenever you run "Build and deploy to pages"

Note that for Joplin server specifically, if you use the Github pages approach, you would need to see if you can set it to use the same domain as your Joplin server, otherwise the sync will fail with CORS errors.