In JSON:API, taxonomy term fields are handled like other entity reference fields.
data below will set the entity reference field
field_entity_reference_taxonomy_term to an existing taxonomy term.
"title": "My custom title"
If you want to create a new taxonomy term and set the value of
field_entity_reference_taxonomy_term to that, then you have two steps:
- POST the new taxonomy term.
- Update the node to set the value returned from the server when you POSTed the taxonomy term (the server returns the UUID in the response when it creates a new entity/term).
On a standard installation of Drupal, there's a nice UI for automatically creating taxonomy terms on the node edit page, but with decoupled Drupal, you have to build this yourself.