Aside from iconic taxa, we used to have more taxa that fit this use case.
Now, “pond scum” is not a taxonomic entity. Cyanobacteria, Chlorophyta, Xanthophyceae, and Lemnoideae can all be “pond scum.” But I will argue that this was a useful taxon. It would have allowed limnologists or others who know how to separate different “pond scum” taxa to see just “pond scum” without having to sort through the entire “Life” taxon.
“All models are wrong, but some are more useful than others” – and non-taxonomic categories can sometimes be the most useful for iNaturalist’s purposes.