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/]:

I suspect this command might stop working soon but I will ask Google for confirmation.

Update: Google confirmed it is going away:

Forum discussion at Twitter.