I started to work on something similar in the past (not turning complete, but cool glyph/ligature substitution). I wanted to make a font that auto-replaced company names with their official logos.
This ended up being far too labor intensive to get right, but it still seems like a cool idea for someone else to tackle eventually.
Pretty cool. macOS's Font Book says "Do not use these font files" and then doubles down with, "Serious problems were found with these fonts and may cause system problems or even crashes if installed," and if you still insist on continuing it will copy the file into ~/Library/Fonts but it cannot be used.
He explains in the article how the file only works with his own hacked version of the HarfBuzz "font engine" (not crazy hacked, so I don't think it counts as cheating, he only increased some sanity check numbers a bit).
I suspect there is something wrong with the font then, perhaps have a look at the console (app) to see if there are any logs related to the warning and log a bug with the fonts creator?
For reference IBM Plex (Mono) fonts that I’ve been using since release do not have warnings and work perfectly, however when I re-encoded them to add ligatures it turned out there were errors with the fonts, macOS warned me and I ignored the warnings, then I noticed higher than expected CPU usage from apps where I had enabled the modified fonts incorrectly.
Very neat. If it supported arbitrary stack-arithmetic, it'd be a really useful terminal font. If, perhaps, the least inefficient desk calculator available.
I do not think the article provides the correct explanation. I'm sure, most of the readers do not use Smalltalk, and they have their own (straightforward) reasons for that. Most probably, why should they care about an old, outmoded, strange and almost dead language? You cannot sell a dead horse.
It is a pity. I could name dozens of its great ideas that are still waiting to be re-invented by the mainstream languages of today.
This ended up being far too labor intensive to get right, but it still seems like a cool idea for someone else to tackle eventually.