Up until recently, the /rate_limit REST API endpoint was not covered by the API's rate limit. While this allowed API consumers to fetch rate limit information whenever they wanted, it was also a potential vector for abuse.
With that in mind, the /rate_limit endpoint is now covered by rate limits. Requests to the endpoint will not consume the primary rate limit quotas for the authenticated user. However, making a very high number of requests to the endpoint in a short period of time will trigger the secondary rate limits. Please follow the guidelines on avoiding the limits and what to do if you do hit them.
These limits are not intended to cause friction for any normal usage of the API. Rather, their aim is to prevent abusive patterns. If you run into any problems with these limits for the /rate_limit endpoint, please contact GitHub Support.