I found a post on Arch forum showing syntax change from brackets to double quotes in 2017
New mkinitcpio config syntax / Newbie Corner / Arch Linux Forums
My pacman log shows 4 instances when mkinitcpio.conf.pacnew was created
2017-10-26
2020-11-05
2022-07-11
2022-12-01
I also found a post on Manjaro forum showingn both mkinitcpio.conf.pacnew and mkinitcpio.conf using brackets
[Stable Update] 2022-07-12 - Kernels, Pipewire, Mesa, KDE Gear, Plasma, KDE Frameworks, Gnome, Python, Haskell - #18 by medmedin
So syntax change from double quotes to brackets was probably November 2020, but I cannot find any comments on Arch or Manjaro to confirm this