Did you know that a font can retire? Adobe retired a group of fonts from their subscription library recently.

Font retirement may happen for a variety of reasons, but is typically at the request of the foundry, when a typeface is no longer available for licensing and distribution. The majority of font retirements are because the font family has been updated with significant changes that necessitate replacing it with an entirely new family. For example, Alverata PE from TypeTogether was overhauled, retired, and replaced by the family Alverata.

If a site is already using a retired font, it’ll continue to display, but once you change the font, you can’t go back to using the retired font.

Here’s a list of the retired ones:
Chemist
Chemist Rough
Couture
Couture Sans
Daniel
Fanwood Text
Intruder Alert
Junction
League Script No 1
Lobster
Luxi Sans
Luxi Serif
Molengo
New Cicle
Nobile
Prociono
Puritan
saxMono
Silkscreen
Simply Mono
Vera Sans

More at https://helpx.adobe.com/fonts/using/retired-fonts.html#what