Why does this I in a black rectangle with a line through appear on my iPad Pro screen, and how to get rid of it permanently?

Since this is happening in Safari several additions could inject content that doesn’t directly come from the websites you listed:

1. VPN or network injection (very unlikely as it would break most secure sites that use TLs)
2. Content blockers (very suspect, try disabling them all, then enabling then half at a time)
3. CSS modifications (less common, but very likely if you use them)
4. Developer tools and experimental features (Settings app > Safari > Advanced)

The developer tools and attaching your iPad to desktop safari and opening web inspector is the tool I use to dissect an odd website behavior. I can not reproduce your issue despite trying, but if someone can show how to reproduce it, I would love to help solve it.