REXX was often used as the scripting language for IVR systems. I haven’t touched an IVR system in 18 years so no idea if it still is.
OS/2 is the primary environment where I had experience reading and writing REXX a long time ago. I was also aware of it being used in IBM mainframe systems, but I had no direct experience there.
I also used REXX on OS/2 systems including doing some development using Watcom VX-REXX which you can basically think of as Visual Basic but with REXX. I released a GUI FTP program I developed with it to the OS/2 BBS but last I looked couldn't find it in the OS/2 BBS archives.