I think the subject matter does evolve, its applications, and the ways that it's taught.
The earliest book on algebra, written by Al-Khwarizmi, was arguably unreadable by anybody but a philosopher. It contained no numerals, just a wall of text. Symbolic equations were invented later. The abstraction of algebra (groups, etc.) even later. Today, kids learn basic algebra in middle school.
The earliest calculus was greatly improved by newer notation. Also, a formal basis for calculus emerged later on.
The teaching of math is a matter of long running debate, since it is regarded as being a widespread failure. Every generation wants to try and crack this problem. Calculus might stay the same, but with varying emphasis on things like problems, proofs, etc.
I would argue that math -- including symbolic math -- is changing again thanks to widespread computation.
The subject matter may not have changed very much, but pedagogical techniques certainly have. A modern textbook is generally much easier to actually read and learn from than a 100-year-old textbook.
There's also going to be minor stuff like you probably don't need to have a section on "how to integrate by using tables of antiderivatives" anymore.
(Not trying to bring down this project, Im all for free and accessible learning and would have liked to have this in undergrad)