Sommething that cannot be customized but before was

I dont know if this is done on purpose but we know resource pack can customize ui but when a new ui part came out like the part when u see the friends and worlds and servers that part wasn't be able to be customized when the new ui part. The ui that cannot be customized is on the photo this happens on any device. I didnt know to say it good at english but photo will help to see which ui cannot be customized.