When can I stop using a baby bath tub?

Across the reviewers we read, most families transition out of a dedicated infant tub between 6 and 18 months. Convertible tubs like the Boon Soak and Skip Hop Moby are designed to support seated toddlers, and r/beyondthebump and r/Buyingforbaby threads cite use up to 3-4 years with foldable tubs like the Stokke Flexi Bath. Sling-only supports like the Angelcare are explicitly rated for 0-6 months.