Somi was never part of ITZY, she left the very month the ITZY team was officially notified (Aug 2018) of who was selected. You don't get told you are selected to debut and immediately quit. You may however quit if you are notified you were NOT selected.

Fan sites often misreport that was on the team because she has a photograph with Chaeryeong at a May 2018 Twice concert, that every trainee attended.. It is well known Somi and Chaeryeong are close friends and would have been hanging out together, however those photos PREDATE the ITZY members selection.

Somi was absent from the Stray Kids survival show (10/2017) which featured the projected next girls debut team, she was available for the show, since her IOI contract and activities ended 10 months prior. Yuna was on that show and on the debut team. Somi was not. It is understandable that Lia was absent as she had just joined JYPe a few months prior.

Somi left Jype Aug 2018. ITZY Members were notified they are going to debut in Aug 2018, they move in together in Aug 2018 and begin working on their debut. So it defies common sense to think Somi was selected and immediately quit. She quit because she was not selected.

ITZY members and Somi have never said that Somi was going to be in ITZY. They would have mentioned it by now if that were the case. They have only said Yuna was added last, never that she replaced Somi, which only indicates that ITZY was initially a 4 person team. (Twice was supposed to be 7 and JYP added 2 at the last moment).