I moderate http://www.speedrun.com/Anodyne , and I'd like to add a column to each speedrun that notes which version of the game it used. I can't figure out how to do this at the /editgame url.

I see there there is a "patch" column at http://www.speedrun.com/hollowknight

Alternatively, how do I search this forum for a previous answer to this question?

You want to create a variable called "Version" or "Patch" and then enter every version as a value.
don't forget to uncheck the "default" checkbox or all your runs will be hidden.

The world "veriable" is not on the /editgame page. Where is the option to create a variable?

bottom of the site, "Extra variables/subcategories" if you need it exact...