Learn English – Single word meaning “becoming lost or forgotten”

I'm looking for a word that means "becoming lost or forgotten," much like how obsolescent means "becoming obsolete" or how moribund means "approaching death; dying." I want to use it in the sense that certain knowledge of the past is becoming lost/forgotten. A sample sentence follows:

"It is a shame that certain traditional customs are [adjectival form of word meaning "becoming lost/forgotten"].

When I say I want a word meaning “becoming lost/forgotten,” I mean to say that the word should connote something that is in the process of becoming less known. In other words, the word I am looking for should mean “in the process of becoming lost or forgotten.” I am trying to be as specific as possible.

Thank you

PS: My example about “obsolescent” is a perfect instance of what the word I’m looking for should be like.