yhanada@ let's chat offline on how it conflicts with arc++ apps. We recently modified this shortcut from alt-shift, so before we make further changes, let's scope out the problem. 

for people not on the existing thread, here's a background doc: http://go/cros-hotkey

basically any shortcut you pick that isn't based on Search is going to conflict with someone.  you might be seeing a report about ARC++ right now, but this isn't the first time IME switch has caused problems.  this has come up multiple times with Secure Shell over the years and we even have a FAQ for it:
  https://chromium.googlesource.com/apps/libapps/+/master/nassh/doc/FAQ.md#how-do-i-change-input-methods
but that solution is still not exactly optimal as Ctrl+Space is not an uncommon sequence to use on the terminal and users are left with picking which feature they care about more :/.

This will be the second time we change it if we do, but I understand the need for it. I don't remember why we didn't go for a search-based shortcut the last time.

 Issue 794093  has been merged into this issue.