More info: it seems removing th_TH[.UTF-8]/Compose is not enough.
Even with the files removed, the Thai XIM is not activated unless
"@im" XMODIFIERS is set to something relevant.
With XMODIFIERS unset, and LANG=th_TH.UTF-8
$ xterm <--No Thai XIM
$ XMODIFIERS="@im=" xterm <--Thai XIM activated
Note that this is true regardless of whether the Compose file
So, Compose file may not be relevant any more.