“Texture healing works by finding each pair of adjacent characters where one wants more space, and one has too much. Narrow characters are swapped for ones that cede some of their whitespace, and wider characters are swapped for ones that extend to the very edge of their box. This swapping is powered by an OpenType feature called “contextual alternates,” which is widely supported by both operating systems and browser engines.

Contextual alternates are normally used for certain scripts, like Arabic, where the shape of each glyph depends on the surrounding glyphs. And they are also used for cursive handwriting fonts where the stroke of the “pen” might have different connection points across letters. Texture healing is a novel application of this technology to code.”

Always able to do it? Yes. Even before OpenType alternates, the extended ligature support in TeX 3.x would have also allowed for this sort of thing.

Why has no one tried it before? Because (a) nobody thought of it and (2) OpenType alternates, while they’ve been around for a while, have not always been supported in the sorts of programs that use monospace fonts (code editors and terminals)

i do prefer neon and argon to the rest, though i see the place for all of them to exist

Input's method seems to be fundamentally very different to this. Monaspace keeps the grid intact and only changes the characters visually (situationally overlaps wide letters to neighbouring narrow characters' spaces). Input just pretends to be monospace in its aesthetics, I don't really understand what's supposed to be special with that.