I have never seen this kind of symbol before and google has failed me.
This notation occurs in Chopin Nocturne Op. 9, No. 1, measure 2 beats 4–6.
Music: Practice & Theory Stack Exchange is a question and answer site for musicians, students, and enthusiasts. It only takes a minute to sign up.Sign up to join this community
According to Wiktionary, the word to denote this is undecuplet or hendecuplet, although there are no articles for those words as of 8 June 2021.