The Short Math Guide for LaTeX is now available on CTAN. It has been significantly updated, contains more up-to-date information and additional notes such as about discouraged syntax and new recommendations. From 17 pages it grew to 21 pages. Furthermore, the source code of the document is now available. It covers all math basics and is a very efficient guide for the smart reader.
Here you can find it: https://ctan.org/pkg/short-math-guide