There is no one particular subset of English you can exclude to simplify its spelling. I don’t really know what you classify as Old English, but whatever Simplified English is, it is highly unlikely it excludes Old English as traditionally defined, as the majority ofthemost commonly used words - italicised here - havesuch heritage. The spelling of such words is probably not the most troubling part of English orthography anyway.
I was saying it's unpredictable if you include Old English spelling and words (which you don't in Simplified English).
The remaining unpredictable words are imported foreign words, usually French.