While there are some web-focussed languages (vs frameworks) that deliberately make it hard for you to write that sort of code, barely anyone uses them.
Not trying call you out on this point, more it made me laugh. Our lead developer strips out characters that break his XML importer (lets just say they are standard common characters, heck, he changes chr(13) to a string so he can switch it back later because he doesn't know how to handle multiple lines in an XML value). People who don't know how to escape inputs / actually handle data properly are in every language. We cringe on a daily basis.
- it is not "input" must be treated, but, so to say - output. It's the destination that matters, not the source. By the time you get the input, you have no idea how and where it will be used. You can tell it right before the output (or, rather, better to call it "use") only. Say, you've got some input that didn't pass the verification and you will have to display it back in the HTML form. And if you already "escaped" it for SQL, it will be malformed. And vice-versa. Moreover, any data must be formatted properly before use, not just something that you tagged as "input".
- the word treated above is for a reason. Because the term "escaping" suggests some certain routine in PHP, which is the actual reason for numerous SQL injections.
I'm working on comprehensive taint analysis for PHP[0], and I'm spending a bunch of time thinking about how to automatically detect those dirty strings.
[0] https://psalm.dev/articles/psalm-3-and-a-half#taint-analysis