WordPress has announced that Webfonts API will not be available for plugins until a later version. It was expected that in WordPress 6.0 Web fronts API will be available for both themes and plugins.
WordPress WebFonts API
The WordPress WebFonts API was first integrated into Guternberg 12.8. It was scheduled to be available in WordPress 6.0 core for wider use. However, WordPress development seems to have paused it for general use and restricted it to themes only. This means a public API for plugins to register and enqueue web fonts will not be available in WordPress 6.0.
The reason why Webfonts API was paused for wider use is that it had not been tested enough for potential risks and there were performance concerns.
WebFonts API Benefits
The WebFonts API was introduced by WordPress to improve the user experience of accessing and defining web fonts.
There was also a suggestion that localizing assets can improve performance and address privacy concerns (for example leaking of IP addresses to font sources. Read about a court case where a website owner was fined for using Google Fonts in a manner that violated GDPR).
Read more about implementing Webfonts API in WordPress.
Also, read about the first suggestion of introducing Webfonts API in WordPress Core.