Unicode is a lot more concerned with what a character means vs how it displays. The box drawing characters that have codepoints are there because they were part of some device's proprietary text encoding, so it's put in a codepage somewhere so a document from that device could be used on something else given it gets converted to unicode.

Pretty unlikely they'd be into adding a new ZWJ pattern for bordering characters since it's awful given unicode's main goal is interoperability. If you copy text out of your terminal, those characters come with it and probably won't render correctly in non-terminal applications (which don't have monospaced text cells), so you'll just see the ? missing character symbol between every letter. (Possibly overstruck onto the previous character depending on handling for the ZWJ.)

I guess the other way to look at this would be "should the background colour be a unicode ZWJ pattern?"... probably not. The border (and colour) is a property of the terminal cell rather than the actual characters IMO. It's not like `X + ZWJ + LEFT-BORDER` has any unique encoded meaning. Cell attributes like the article is talking about IS the less archaic way of doing this. ZWJ patterns are a lot more archaic and cause a lot more issues. Overstrike is fine, if it's being down with cell attributes (meaning, moving the cursor overtop of a cell, and stamping a new character on it instead of replacing it. Exactly how mintty does it.)

> Unicode is a lot more concerned with what a character means vs how it displays

This was only true before emojis. Unicode introduced specific code points purely to alter visual display, such as skin-tone modifiers, etc. Now that the mission was already diluted for emo-fluff, you might as well add actually useful fluff.