I absolutely love ShareLaTeX. It's one of the few products that is kind of niche but does one thing and does it really well. I definitely get $8 a month worth out of their service. Having used MacTeX and similar products, ShareLaTeX is just a much smoother user experience. Not to mention their documentation is great and really gets to the point of what you are often trying to do. I've used their service on the last three research papers I've written and have been sharing the platform with different professors I work with.
(I sound like a shill, but check my comment history — I'm not affiliated with them. Just a happy customer.)
Personally I just can't get used to using a slow web app. I much prefer working in a native editor or IDE (like TeXStudio). I still use Sharelatex in a pinch, when I'm another machine or something, but I breathe a sigh of relief when I'm back on a native app.
One of my favourite feature of Overleaf (another cloud LaTeX Editor) is that I can use git to work on my cloud project on my local machine. It seems to me the best of both worlds.
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.
(I sound like a shill, but check my comment history — I'm not affiliated with them. Just a happy customer.)