How do I maintain API versions?
One approach to version the API is to add a version number to the URL.
So, for example your Views REST display URL /rest/taxonomy-list would become /rest/v1/taxonomy-list or /rest/v2/taxonomy-list
You could duplicate the existing Views display to adjust the fields and new URL in a new REST display.
For custom REST API plugins, you can extend the existing RestResource and give it a new URL, e.g:
use Drupal\my_module\MyRestResource;

/**
 * Sample Rest Resource
 *
 * @RestResource(
 *   id = “my_rest_resource_v2”,
 *   label = @Translation(“My Rest Resource (v2)“),
 *   uri_paths = {
 *     &quot;canonical&quot; = &quot;/api/rest/v2/endpoint”
 *   }
 * )
 */
class MyRestResourceV2 extends MyRestResource {

  // Extend and override functions here 

}