Hello,
there is no reason to exclude any posts from the WPFTS index. That’s why.
WPFTS completely controlling the part of WP_Query functionality responsible of full-text search. In case we exclude some posts from the index, they will not be searchable anymore.
In our FB dialogue, you said that it will be extra speed in case index will contain only these posts, which you planned to search in. Correct. However, some WP modules may require a search for other post types anyway (for example search widget just above any WP admin table allow you to filter post lists by text query) or even internal functionality will require that.
You should not worry about a too big number in the “N posts in index”, it just meant everything is under control of WPFTS.
Thank you for your message!
