Escaping user input is ridonkulously hard

A type system isn't going to save you from users submitting all kinds of potentially different encodings. Which also depends on what kind of user input is being handled: Is it OS-provided UI? Is it something being sent to a service accessible on the internet? Is it from a CLI? Is it from a file? Context matters for the potential space of what kind of data you might be operating on, which could require different ways of either knowing what kind of data you have based on having more control over the input versus having to detect stuff (or be told, correctly) from highly arbitrary things like reading from a file. All of that is external to the type system, and requires doing something before you can tag it with the correct type. Some languages might attempt to detect this stuff for you, but that could potentially be considered a language defect if it's hard to detect what a string is without having other input telling you what that string contains, such as a header in an HTTP request saying that it's UTF-8.

"A type system isn't going to save you from users submitting all kinds of potentially different encodings."

Yes, it is, because you give that a type that indicates you don't know what the encoding is, like RawInput or something. You then can not pass this type to any other function that doesn't explicitly call for that type. If you have some function that accepts it, blindly casts it to UTF-8, and slams it out into a file, well, that's not the type system's fault [1].

Of course a type system won't prevent you from still just being wrong or writing bugs; nobody promises that, not even the formal methods advocates. But it will prevent you from just accidentally blindly shoveling it out somewhere it doesn't belong without ever examining it or thinking about it.

I think you may be believing in a popular myth about strong typing systems, that they are designed to somehow prevent bad data from coming in to your system at all. You correctly identify that as impossible. But what strong typing systems can do is force you to deal with the fact that bad data may be coming in. On the outside, you have the chaos of, say, a bag of bytes that may or may not be JSON. On the inside, you have a "type SomeStruct { int a; int b }". A strong type systems forces you to write some sort of adapting code between those two, and guarantees that the result of that adapting code will be only and exactly the type that comes out of that adapting code, no "whoops, sometimes this dynamic code just returns a string, or maybe a network socket, or who knows what". Nothing can prevent your HTTP API from receiving a JPG of an anime character instead of JSON specifying a user to delete, but a strong type system can make you deal with that immediately and fully, instead of garbage data of indeterminate type floating through the system for an indeterminate period of time.

[1]: Also note there are a lot of "strong type systems" in the world that still fail to take advantage of their own capabilities and let bare string types and such float around too much. There are reasons why libraries must support the lowest common denominator; a file is a series of bytes with no further constraints, so the lowest level API has no choice but to accept that, but higher level APIs should more often take more restricted types. That strong type systems can save you from this doesn't mean they all do. I have a number of wrapper types in various languages just to add these guarantees to my programs not provided by the underlying libraries, though I also have some code that just wraps the underlying libraries that can't help but correctly take raw bytes at the lowest level.

The biggest problem appears once you start wanting to combine such strings.

Say a user inputs some raw text in a form that is intended to be the title of a button in a HTML form that will be sent in a JSON file to be stored in a SQL db. The expectation is that you can later retrieve this HTML snippet from the DB and display it on the screen.

You have to first escape the raw text from the user so that it can be safely used in HTML - so you will go from user_input_string to html_pcdata_escaped_ user_input_string. Then you compose a bit of HTML that contains the button and this part; let's say you store it in some HTML DOM object. Then you want to send this HTML object as JSON, so you have to know to convert html_pcdata_escaped_ user_input_string into json_string_escaped_user_input_string - but that loses type information which may hurt us later, so maybe we want to actually store it as json_string_escaped_html_pcdata_escaped_user_input_string. Then, if we want to use this as part of an SQL query string, by the same considerations, we want to put it in a mysql_like_filter_escaped_json_string_escaped_html_pcdata_escaped_user_input_string - which is getting really ugly, and easy to mess up.

Of course, the order of escaping matters, so an mysql_like_filter_escaped_json_string_escaped_html_pcdata_escaped_user_input_string and a json_string_escaped_mysql_like_filter_escaped_html_pcdata_escaped_user_input_string are different things that need to be decoded differently (of course, for SQL in particular we could use prepared queries instead).

Also, we can't ever concatenate this with any other string-like type until perhaps the final use point (such as sending a query string to the DB), since we need to remember which part of the string is escaped in which way, and for what types of uses it is safe (an HTML-escaped string may still contain SQLi or JSON injection).

The point is that even with proper types, this is not easy to manage or fix.

It also requires quite advanced type systems to be able to use these in normal contexts - say, you want to store several such strings with different provenances in a Map or Set or even List, without "forgetting" the provenance.

As far as I'm concerned, "the point is" that what we're describing here -- keeping track of the type of each piece of data -- is the best way to think about this class of problems (as opposed to talking about "sanitising" or "safe data", for example).

It's then up to us to decide how to best make use of the type system of whatever language we end up implementing it in (or, indeed, to treat the ability to deal with this well as a requirement when we're choosing a language).

For me, effects like "we can't ever concatenate this with any other string-like type" are desirable features, not problems with this approach: either it's possible to convert both strings to a common form, or I shouldn't be trying to combine them.

Sure - I'm just saying that the article is right that this problem is difficult, not easy, and that it doesn't get significantly easier if we accurately keep track.