OMG NEVER! 😲
As a very young child, I remember visiting my gran and my mum dunked her biscuit in her cup of tea, my gran gasped in horror, and told us that dunking was extremely impolite and should never be done, especially not in company.
This has stayed with me for over 50yrs, and I've never dunked, that's to say that I may have done it before being told it was rude, I can't remember, but I've definitely not done it since, and seeing other people dunk gives me the ick 🤮
One caveat though... bread/toast in soup is different, that's allowed 🙂