Google has removed the related search operator from its “overview of Google search operators” help documentation. The related search operator would show you pages that are related to a specific URL.
It still seems to work for me, but it was removed from this help documentation page. This is what it looked like a month or so ago:
Here is what the page looks like now:
When I test the command, it seems to work currently. For example:
[related:https://www.google.com/]:
[related:https://www.seroundtable.com/]:
