Do you have a source for that?
Page 14 of ToA is inconclusive; it strongly implies Ubtao is gone and forgotten, but also has the phrase "Ubtao himself is no longer popular" when it would have been easy enough to either change to "Ubtao himself is no longer worshipped" or even "Ubtao himself is no longer popular or worshipped".
Personally I think it is bad setting design to categorically state something never, ever happens. Setting guides are supposed to inspire ideas, not rule them out.
"Hey wouldn't it be cool if there was a Chultan group trying to entreat Ubtao back to protect the Chultans once more?"
"Sorry, you can't do that. Absolutely nobody worships Ubtao any more."