This plugin adds the ability to use name_first_letter in site setting external system avatars url.
The default value for external system avatars url is
/letter_avatar_proxy/v2/letter/{first_letter}/{color}/{size}.png.
This plugin allows you to use
/letter_avatar_proxy/v2/letter/{name_first_letter}/{color}/{size}.png
to have the avatar be the first letter of the name rather than username. If there is no name, the username is used.
You must change the site setting for the plugin to have any effect. It affects only users with the default letter avatar, not those who have uploaded an image avatar.
Install like any other plugin.
This plugin was working in August 2022. I believe that it is unlikely to get broken by future changes to core.