Background
The Growth-Team works on decreasing the number of user_properties rows GrowthExperiments extension occupies (T304538). Deletion of user_properties rows has to be done carefully (because of the risk to break something), but there is no pre-existing script to do that job. This made the Growth-Team to create single-purpose scripts to do the deletions.
Problem & Solution
To avoid more and more custom scripts existing, let's make userOptions.php capable of deleting user options.