This is only for taxa that have been marked “complete” (said to contain all its known descendants). If you click where its says “Show uncountable species”, it appears.
It’s intentionally not shown in this view. Maybe that link should be renamed something like “Show uncounted taxa”. Or always appear, but be formatted differently.
Marking a taxon as complete:
loarie: All this does is trigger the ‘trends’ tab and ‘TOTAL SPECIES OBSERVED’ UI, and the ‘all taxa added’ decorations in the taxonomy tab on taxon pages (e.g. https://www.inaturalist.org/taxa/26036-Reptilia) for the point of attachment and all covered taxa coarser than rank species. It should only be selected if a taxon framework has all extant covered taxa (excluding hybrids) added. Also note that its arbitrary how resolved the internodes in a framework are (e.g. a framework can have tribes or not have them) - it should be described in the taxon framework. So ‘all taxa’ restricted to the desired resolution (e.g. a framework missing tribes can still be complete if thats how the framework wants the branch to be)