The piece of square plastic with a letter, number, or symbol is usually called a 'key' but is ... because you may end up doing more harm than good and damage the keyboard further.

Home and End key placements or reduced sizes. This can be critical and mainly affects the fast touch typist. A desktop computer keyboard can be replaced at any time with a high-quality keyboard ...

TL;DR: Apple has updated the mute key on the MacBook Air M4 to feature a speaker symbol with a strikethrough ... it was an oddity with the keyboard, and a slight pitfall for some Mac owners.