The ligatures for != -> etc. are a terrible idea, imagine someone reading a programming language tutorial or a snippet and then actually trying to type stuff rather than copy-pasting.
This is like saying you shouldn’t render latex. I like my ligature font, it makes nicer to read code for me, not terrible at all. Your programming font is a personal choice, i guess when publishing a tutorial you need to consider which font is appropriate, but that doesn’t make anything terrible.
Ok. However, I publish all my code in plain text. Arguably this this is the normal way code gets distributed, unless you edit in word... The choice of font, syntax highlighting, editor, etc are personal choices, what is terrible for you might not be terrible for me.
You publish your code in plaintext, it goes to github where someone reads it through some fancyfying code formatter that github applies to it and which might not even do the same ligatures as your editor. I'm not saying this is a dire problem, or a problem with you or even this specific font - rather the idea of ligatures in program source code to begin with, somewhat similar to using code identifiers (as opposed to literals or comments) outside of ascii-compatible characters of unicode.
That's not a very good thing, especially considering that there are (probably ?) actual unicode characters '→' and '≡' that are not valid syntax for -> and === in languages that utilize the later.
Good for who? In an IDE the person reading them knows what don’t they are using, and so won’t get confused. The IDE itself knows what language the code is in and will highlight invalid syntax.
I'm sure they're very rewarding for the author/editor - but the cost is externalized onto unversed readers, this is generally considered bad practice in other areas of programming.
The good thing about ligatures is that they only affect how the text is displayed, so people who don't like and use ligature fonts won't encounter them.
The exceptions may be, as you said, online tutorials etc. with their own fonts and formatting, but I think that's more a part of the problem of bloated website design overriding browser-wide preferences.
For me ligatures are less important than syntax highlighting, but I do find them helpful - especially the greater-equal and smaller-equal signs are much more readable with ligatures, and that also applies when I read other people's code in my editor.
Not really, too often on the web it is the font decided by the author or an intermediary - the above page is case in point or they wouldn't have been able to demonstrate it as I have never explicitly opted-in to ligatures at any point.