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.