The Brits I know...many of them...use it a lot! Example: Someone brings a gift and the host replies: 'You needn't have! ' It's almost like saying 'thank you', but it has the effect of complimenting the giver in this sentence. Or someone does something for you - something kind - and you reply: 'You needn't have'. You are thanking the doer of good deeds but also very subtly complimenting them. Other ways they use this: Janet is complaining about the price of a dress. Your reply: 'You needn't buy it.' In other words, you don't have to buy it if it causes you stress. If you leave out the negative, it becomes 'you must' . Example: ' You need to leave on time or you will not catch the train. South Africans use it a lot. '

'Need' is a semi-modal verb, but the modal form is falling out of use. The 'needn't' form can sound quite dated and formal, especially to American English speakers. You might be interested to look at this discussion: https://www.italki.com/discussion/216919 If you read the thread and still have further questions, feel free to ask.

"Needn't" is acceptable English but is quite dated. People prefer to say "don't/doesn't need..."

Interested question, I want to know answer