This forum includes an article, FAQ: “Notepad++ is a Text Editor, not a Word Processor”, that goes into this.

Many of the comments on the superuser and stackoverflow links you provided are saying the same thing. That said, people can coerce Notepad++ into behaving where some of the markdown syntax displays in ways similar to how they would display if you had a full markdown processor.

Part of the problem within Notepad++ is that it does not support the concept of a preview window. Many people have asked for this, mainly so they could type HTML and instantly see the results. These requests/suggestions have always been rejected as Notepad++'s focus is the underlying text that then may be processed by other applications such as a web browser or markup/markdown processor to render the text into what end-users see. Notepad++ is not supposed to be a WYSIWYG editor.

That all said, it’s cool to see that Notepad++ can be coerced into displaying strikethrough or strikeout text.

1. https://superuser.com/questions/191430/strikethrough-in-notepad#:~:text=Enable Support for Strikethrough in Notepad++
2. https://stackoverflow.com/questions/8065138/how-can-i-use-rich-text-in-notepad#:~:text=Enable Support for Strikethrough in Notepad++

Also note step 4c should be as below:
Open: ~~
Escape: \
Close: ~~

If the goal is to highlight instances of ~~ pairs in a text file then your idea does the trick. If you are in an environment, such as a legal office, where your text files are already using ~~ pairs then it makes perfect sense.

As plain text does not have styles or attributes it is better to use a markup language that supports styles or attributes. This message forum’s markdown language supports strikeout for example by typing ~~strikeout~~ which the markdown translates into the HTML <s>strikeout</s>. Even people using specialized web browsers that render web pages as plain text will see the strikeout as intended as it’s based on well known standards. Blind people use these web browsers along with text to speech software.

I already had the Lynx web browser installed as part of accessibility testing and see that strikeout text is displayed as [DEL: text with a strikeout through it :DEL]. Thus a blind person will still understand that the web page includes some deleted text that is part of the document.