So there is this one way to get technically free Robux, though it's at the expense of your time and maybe some user data too. Microsoft has this rewards program (?) for using its services. By using bing and by doing activities, you get these points which you can then use to redeem stuff once you've racked up enough. One of these rewards is Robux. Other rewards include also Minecoins for Minecraft, overwatch coins, and you can even get gift cards for starbucks, tesco, argos and stuff. But the amount of points you need to redeem for something may vary depending on where you are. Also, it sometimes take a long while for the site to stop giving errors when redeeming, but for me it always ends up working eventually.
I hope this is helpful! <3