The Unified Index is a temporary copy of some of your data, optimized for quick retrieval.
Step 1
tiki-admin.php?page=search -> General Settings -> Don't index non searchable fields
Indexing will skip adding all tracker fields that are not marked as "searchable". This will free index space but also make it impossible to use those fields in search index queries.
Preference name: unified_exclude_nonsearchable_fields
Step 2
Then, go to the list of fields in each tracker, and toggle the value for the "Search" column.