WPGraphQL Tax Query
This extension exposes arguments to PostObject connections allowing for posts to be filtered by taxonomy/term fields.
Why isn't this in the core WPGraphQL plugin?
The shape of the input fields is still under debate. The ability to filter PostObject connections by terms will exist in the core plugin in some form, but how it should look is still up in the air.
This plugin exists as one implementation, but how it looks in the core plugin will likely be different.
In the mean time, this is a good option if you need that functionality.