After going through and cleaning my disk with old things that I didn't need anymore, I came across the iOS DeviceSupport
folder in ~/Library/Developer/Xcode
which was taking nearly 20 GB.
A similar question has been asked before, but since then many things have changed and I would like an up-to-date answer.
As long as I have the version I use for testing, can I delete the older/unused versions without breaking anything?
The ~/Library/Developer/Xcode/iOS DeviceSupport
folder is basically only needed to symbolicate crash logs.
You could completely purge the entire folder. Of course the next time you connect one of your devices, Xcode would redownload the symbol data from the device.
I clean out that folder once a year or so by deleting folders for versions of iOS I no longer support or expect to ever have to symbolicate a crash log for.