waterway=stream_end is the equivalent of noexit=yes, that is it indicates the end of a waterway so that QC checks will know that the end of the waterway is intentional and not a mapping error. =inlet / =outlet are physical things and if there is a soak hole the correct tagging should be something like natural=soakhole. Although it would need a better name than “soakhole”, because this name is also used in NZ for constructed stormwater management infrastructure Everything You Need to Know About Soakholes - Drainage NZ.