At least as far back as Python 3.1, the description for Template strings (section 6.1.5 in version 3.6.4rc1 docs) starts by differentiating what Template strings do, as:

Instead of the normal %-based substitutions, Templates support $-based substitutions, using the following rules:

Since this immediately follows a section describing the "Custom String Formatting" and the "Format Specification Mini-Language", which does a type of substitutions that is {} based, rather than % based, it is hard to grasp exactly why %-based substitutions would be considered "normal". Of course, I know why, due to the % operator, but for someone just reading through chapter 6, it is a reference that raises the mental question "Huh? What is normal %-based substitution? Are Templates abnormal, if %-based substitutions are normal? What did I miss? The previous section was about {}-based substitutions? Are they abnormal, too? What are normal %-based substitutions, anyway?" rather than helping to describe what Templates are and do.