When fonts are being developed there are a variety of gylphs, ornaments, widgets, etc. That need to be added into the font family but there are various type designers who leave them out because they don’t want to be bothered with the design.
My favorite font I have ever been able to use is Avenir Next, and they took it upon themselves to go through and design special characters, greek letters, etc. Because when it comes to a good design you don’t leave out possible clients.
Although people choose not to design them based on if they would work with their type design, sometimes they don’t. Specifically the way their typeface is designed may not be able to produce a special character or ornament; it could be unreadable, so why make it?
It is always interesting how fonts, and typefaces are designed and what is chosen to be designed and what isn’t based on their structure.
Typography Fall 19 