When using REST to gather the JIRA fields, the value from searchable will return true even when you choose None for the search template.

How to reproduce

1. Create a custom field (i.e Number Field)
2. Click on Edit (from the cog item)
3. Select None for the Search Template
4. Trigger a REST as http://JIRA_ADDRESS/rest/api/2/field

Results

Searchable will be returned as True as:

        "id": "customfield_10400",
        "name": "Testttt",
        "custom": true,
        "orderable": true,
        "navigable": true,
        "searchable": true,
        "clauseNames": [
            "cf[10400]",
            "Testttt"