Stack Exchange network consists of 175 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.
The term is used in shops and stores to welcome customers. It seems impolite to ignore the greeting.
It isn't impolite to ignore いらっしゃいませ and most Japanese don't respond to the greeting.
However if you want to respond to it, you may say こんにちは and give a shallow bow.
Required, but never shown