That isn't the case. Firstly, you need to take a look at the browser stats for the site that you are working on. It may be that in your case you have a very high percentage of old IE users, and so for you, flexbox isn't an option. Even then though, you may be surprised.
If you use flexbox as an enhancement, to get really good alignment of UI elements for example, then you can often take advantage of it to make the experience really great in newer browsers while not locking out old ones.
I would suggest taking a look at Zoe M. Gillenwater's talk. She works for Booking.com. Are they locking out old IE users? Of course not. So she demonstrates how to enhance responsiveness with flexbox. https://www.youtube.com/watch?v=_98SE8WUvLk
If you use flexbox as an enhancement, to get really good alignment of UI elements for example, then you can often take advantage of it to make the experience really great in newer browsers while not locking out old ones.
I would suggest taking a look at Zoe M. Gillenwater's talk. She works for Booking.com. Are they locking out old IE users? Of course not. So she demonstrates how to enhance responsiveness with flexbox. https://www.youtube.com/watch?v=_98SE8WUvLk