Github starts deprecating the anonyms Gist creation to stop spam. 

In 30 days, we'll be deprecating anonymous gist creation—a decision we made after a lot of deliberation.

We'll deprecate anonymous gists using the following timeline:

March 19, 2018 19:00 UTC (11:00 am PST): Disable anonymous gist creation via the web and API.

But you need not to worry if you have created Gist earlier as the current anonymous gists will remain accessible.

To learn more about creating gists, check out the documentation.