With Relevanssi Premium, you can use the relevanssi_match hook (see this documentation) for shutting down specific custom fields in the search based on a query parameter.
With the free version, the same method cannot be used, as the customfield_detail field that lists which custom fields match the search term is a Premium feature.
With the free version, you can do something similar and check the desired custom field value with get_post_meta() within the relevanssi_match function. That takes some care, as you can end up doing a ton of extra database queries (you want to do a single database query to fetch the custom field value for all posts at once and then use that data).
Or, you can do meta_query, that’s a fine approach, too.
Niiice! I have Relevanssi Premium, so I can make use of the customfield_detail detection. Will try this out. Thanks for your quick response. Stellar support as always! 🌟