Historically, up to Draft 7, the $ref keyword overrides sibling keywords. This resulted in many schema developers being forced to wrap instances of $ref in an applicator like allOf. However, that is no longer necessary in 2019-09 and later.

For example, consider this schema:

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "allOf": [
    { "$ref": "https://example.com" }
  ]
}

As long as there is no $ref sibling to allOf, it can be extracted as follows:

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "$ref": "https://example.com" 
}

Of course it applies if there are multiple branches in the allOf, etc.