That is the generic argument against extension methods wherever they appear. They are very confusing, until you get used to remembering their existence, then you know what to search for.

Also, how about IDE support? I would argue it is at least as important as the feature itself.

This is mentioned in TFA.

But I agree about "this" being similar concept

The "this" that GP mentions is for for extension methods similar to the article to be regcognizeed as such (never done if the annotation isn't present), they're written such as:

static int Foo( this Bar bar, int biz) => bar.xyz+biz;

Bar bar = new Bar() { xyz=123; };

int result = bar.Foo(456); // Works even if Foo wasn't a part of Bar initially

Elsewhere annotations(called attributes in C# land) can often be used in the same way as in Java to document serialization behaviour,etc. So in practice there aren't much differences apart from how library authors haven't embraced it entirely as much for automagic bindings (partly due to philosophy, partly due to other facilities being more appropriate).

The definition that includes it is so broad that it loses all meaning, it would make "private", "class" or "boolean" also annotations.

I mean it's fine to not like annotations or the decision to use them, but that another language does things differently is almost a tautology and hardly worthy of note. IMO, reasonable people will disagree.

  "use strict";

  from __future__ import annotations

  {-# LANGUAGE TemplateHaskell #-}

In JavaScript there is .js, .ts, .jsx, and .tsx. I hope that doesn't become a lot more, but for low amounts of variation, it's reasonable.

I think Haskell is a good example of what happens when a language supports too many extensions. Why are there so many? How do people deal with them all? Needing to pick another file extension seems like pressure to do this more sparingly?

"This looks odd to me so no one else can possibly understand it"is not a rigorous argument.

Both Scala and C# have this. I've used it in my own code to clean things up. It's fine.