There's a regression in Xcode 26.x (26.0 ... 26.5 beta) where xcodebuild is not able to download platforms when it is behind proxy.
$ env | grep PROXY
HTTP_PROXY=.....
HTTPS_PROXY=.....
$ xcodebuild -version
Xcode 26.5
Build version 17F5012f
$ xcodebuild -downloadPlatform iOS -buildVersion 23E244 -exportPath ./test
Finding content...
iOS 23E244 is not available for download.
Xcode 26.x GUI is able to download from behind proxy though.
The last known Xcode which used to be able to download platforms from behind a proxy was Xcode 16.4.
$ xcodebuild -version
Xcode 16.4
Build version 16F6
$ xcodebuild -downloadPlatform iOS -buildVersion 23E244 -exportPath ./test
Downloading iOS 26.4 Simulator (23E244): 0.0% (66 kB of 10.6 GB)
The relevant feedback ID: FB22125784