Yea personally I found out about ZARD a while back.
Just recently I was rewatching episodes and searched up a couple other artists.
For example, Miho Komatsu sung the OP3 of detective conan. 2006 was the last time she released a song. 2009 was the last time she updated her blog. According to wikipedia she never announced her retirement or hiatus but my guess is she's retired given the time that's passed.
And I found out about her through the covers which La PomPon made for 2 of the earliest OP. One of them is OP 3 "Nazo" and the other is "Unmei no Roulette Mawashite" that you mentioned. Turns out La PomPon disbanded 3 years after their debut in 2015 (meaning 2018). Kinda sad to see artists come and go but I guess that's just time passing by. You wonder where these people go really.
We watch the show and hear their song/cover for a couple of episodes and then never again. They disappear into the background.
Sad that ZARD's career was cut short (My opinion) but I guess we can never have our way with things.
