It looks like returning false from QWebEngineCookieStore::setCookieFilter(...) does not filter cookies anymore with Qt 5.15, while it works fine with Qt 5.14.2.
- Remove ~/.local/share/QtExamples/simplebrowser/QtWebEngine/Default/Cookies (or the equivalent on non-Linux systems)
- Patch simplebrowser to reject all cookies:
- Run simplebrowser youtube.com with Qt 5.14
- Run sqlite3 ~/.local/share/QtExamples/simplebrowser/QtWebEngine/Default/Cookies 'select * from Cookies' and observe how there are no cookies
- Repeat with Qt 5.15 and you'll see various cookies saved to the database.