Variable attribute is_searchable_choice is not considered on Service Portal; setting it to false does not have an effect, as it will always be on regardless of configuration.
Steps to Reproduce
Add a variable type Lookup select box OR Select box into a catalog item.
Set attribute is_searchable_choice=false.
Open the catalog item page in Service Portal.
Note that the variable will be searchable, unlike in the platform UI.
This attribute is intended to help the transition towards the searchable select box and is therefore working as designed.
Related Problem: PRB1276641