List collector is only available for Catalog Item variables.
The slushbucket is a feature only available on the platform side as such. Similar functionality within Service Portal is available if you use List Collector variable types and applies for catalog item variables of type List Collector. Main difference between platform UI is the List Collector variable will display in Service Portal as a choice list where multiple values can be selected whereas on platform UI it displays as a slushbucket
All versions that support List Collector variable type.