The most similar website is probably KnowledgePoint, a site jointly run by some UK based INGOs:https://knowledgepoint.org
Their focus is a bit broader, i.e. not only WASH, but other than the moderators not so many people have participated lately.
mailing lists on Dgroups
are a very large exchange group mainly related to water supply (most active group is the "sustainable ground water development").
Mostly sanitation & hygiene focussed, but quite active are the SuSanA discussion forums:Sustainable Sanitation Alliance Discussion Forum
If you have an account on LinkedIn (and are aware of the privacy concerns related to using such commercial sites), the WSSCC's community of practise
might also be interesting.
There also used to be a discussion forum on watersanitationhygiene.org
but it hasn't been active for some years and seems to have been disabled recently.