Axolotls, olms, and sirens stayed in the juvenile stage of salamanders, and the latter seem to be well established in that form. I’ve seen one example of a bullfrog tadpole that didn’t metamorphose and kept growing to the size of a large adult, but it would take a whole bunch of unlikely coincidences for multiple individuals to do that at the same time and have the capability of reproducing.
I suppose caecilians could be an even older example of this than sirens…