tell them to get a life and that it doesn't give them a larger dick because they backed, it just gives them larger velocity and cool headphones, along with other stuff that may or may not be cool. Sure they helped develop the game, which is great, but there's a difference between investing money (only takes money and zero work) and doing the work to make the game happen. Without the devs, the amount of backers and pledged money would make no difference.
Backer items won't be available anymore, but similar alternatives will be made eventually. Backer items was also the absolutely last time pixeltail offered exclusive items, according to what mac posted on this forum two years ago, so expect it to stay like that.