That change has been tipped to be a "0.1mm slope on the display edges," which Weinbach says will make using screen protectors difficult. That makes sense given a previous leak from Weinbach that suggested that Apple's adding curved edges to the back and sides of the phone, which would require other changes around the phone to compensate for the new shape.
One thing to note was that this existing leak was for the iPhone 15 Pro, but in this new tweet, Weinbach doesn't specify a model. It sounds like he's referring to all four expected new iPhone models, which would match with what other leakers have said about the whole iPhone 15 series adopting the new look.
Unlike many leading Android phones, iPhones don't have pre-applied screen protectors, so anyone who wants one will need to apply it themselves. Apple may well argue they're not necessary since the Ceramic Shield glass that recent iPhones use is pretty tough. Still, as any unfortunate iPhone user will tell you, even Ceramic Shield displays will scratch and shatter under the right circumstances, so adding an extra protective layer can be a wise move.
Hopefully, the third-party iPhone screen protector manufacturers hearing this rumor now will have time to adapt their products for the iPhone 15 series, assuming what Weinbach has to say is accurate. It could prove difficult, especially when it comes to making hard, tempered glass protectors, but they've at least got plenty of time. If Apple keeps to its usual schedule, the iPhone 15 series will debut in September, another seven months off.