No, there is no setting or feature parameter required to support the multi-picklist field. You just need to add field mapping for the picklist field on the Field Mapping screen and give read/edit access to that field in the public access settings of the site.
- If you want to add a multi-picklist field on the Account Update Form, please follow these steps: https://satrangtech.freshdesk.com/support/solutions/articles/17000084835
- If you want to add a multi-picklist field on the Contact Update Form, please follow these steps: https://satrangtech.freshdesk.com/support/solutions/articles/17000084836
- If you want to add a multi-picklist field on the Lead Update Form, please follow these steps: https://satrangtech.freshdesk.com/support/solutions/articles/17000122993
P.S. Discover Update Client Details, the easy way to keep client information up to date in Salesforce. Find it on AppExchange - https://appexchange.salesforce.com/appxListingDetail?listingId=a0N3A00000EcsATUAZ