Async Json Schema Validation Keywords (SVK)

Set async:true into the keyword object

const extend = {
  keywords: {
    checkUser: {
      async: true, // <<---
      validate: (field, value) =>
        simulateAsyncFindUserCall({ [field]: value })
          .then((items) => (items.length === 0)),
    },
  },
};

And set $async:true into the json-schema as well

const schema = {
  $async: true, // <<---
  type: 'object',
  properties: {
    ...
  },
};

Read more about Asynchronous Validation on the official epoberezkin/ajv documentation.