FWIW, it's not just ShareLaTeX... I learned LaTeX a few years ago and through the tutorials and resources I used I learned the `$$` syntax. I'm a very infrequent user, but this is the first I've heard that it was depreciated at all, let alone 20 years ago.

I do support OP's thoughts that a 'modern' LaTeX solution like ShareLaTeX should encourage best practices though.

Out of curiosity, what should I use instead? And what are some tutorials that promote more modern techniques?

\begin{equation} ... \end{equation}. Finding modern resources to learn LaTeX (and indeed TeX for macros etc) has been a perennial difficulty for me!

Or \[ ... \] for unnumbered equations.

I would use

  \begin{equation*}
    ...
  \end{equation*}

for consistency and ease of switching between numbered and unnumbered.

It's true, finding modern LaTeX tutorials is even harder that modern c++. It shares many of the same problems.

We put together a LaTeX course (in german) for physics student every year and keep it up to date with best practices as much as possible: http://toolbox.pep-dortmund.org/files/archive/latest/latex.p...