The article is talking about symbols that were lost. The sentence should be interpreted as "these [symbols], for Insert (now gone) and Delete (still with us)". The photo is of a symbol for 'insert', an 'a' with a caret above it.
It probably makes sense with a silent linear keyswitch for users who are used to the obvious acoustic feedback of a typewriter.
Since cheap modern MCUs have programmable ADCs, you can wiggle an output pin and experiment with different sounds. I've tried something a bit more "tinkle" like than a flat "beep" and it has its charms, but it could go a whole new place like different tones for special keys (simulating the clunk of a shift or space bar) or even keys that you could subtly differentiate by sound to tell if you're off alignment.
"Key beep" sounds horrific. :D