!!! Overview
[{$pagename}] is a [OAuth 2.0] parameter used to prevent [Cross-site request forgery] and [Best Practices] would be use a [CSRF Token]

Some folks recommend this be a [Digital Signature] and stored within the [browser] [cookie]

[Encoding claims in the OAuth 2 state parameter using a JWT] points out some recommendation on use of [{$pagename}]
                

[{$pagename}] is a form of a [Nonce]

!! More Information
There might be more information for this subject on one of the following:
[{ReferringPagesPlugin before='*' after='\n' }]