Default interface implementations were introduced in C# 8.0, along with modifiers on interface members.
However, protected interface members can be called from derived interfaces only, not derived classes.
You can read the original specification for the new interface-related features and some of the design discussion and decisions here:
Here are two more related articles:
https://www.talkingdotnet.com/default-implementations-in-interfaces-in-c-sharp-8/
https://jeremybytes.blogspot.com/2019/11/c-8-interfaces-public-private-and.html