Well that's interesting, you never stop learning something
So that option doesn't appear in any other version yet, only Designer Pro X.
Yet the Help file very confusingly states:

Standard keys / Classic keys

At any time you can press Standard Keys to set all the command shortcuts back to the default settings for Xara Designer Pro X.
Alternatively press Classic Keys to reset to a keyset that was used by early versions of Xara Designer Pro X.
Now when did I miss an earlier version of Designer Pro X that used different shortcut keys to the version I use now !?!

I've tried CTRL+8 in all versions back to Xtreme Pro 4. Doesn't work for me.
Do you know which version/s of Xara software had CTRL+8 as the key presses for full screen?