Writing this here so that it doesn't get lost.

@Hackwar has stated that he doesn't want the stemmer to be enabled by default as there is a performance hit and that users should make a conscious decision to enable it.

However there is no option in Joomla 4 for the language stemmer.

Apparently this field is the one that applies to the stemmer

I have no idea how any user is supposed to guess that or that by selecting it there may be a performance hit.

There is no mention of any of this in the help docs as it still refers to the two fields that were used in J3 for the stemmer

Finally any site that upgrades from j3 and was using the stemmer will lose their settings on upgrade as the field name used in J4 is different to the field name used in J4. I couldnt find any mention of this in notes for upgrading.