The Bosworth-Toller Anglo-Saxon Dictionary
is perfectly adequate to demonstrate that the Old English ancestor of the Modern English verb to say was a rather "strong verb" Wikipedia
Even if to say appears in Modern English to be a regular (weak) verb, it is not.
say, says, and said
There are different vowel sounds in say and said, making to say a strong verb.
While one might question the use of "i" and "y" to represent different sounds, there can be no question that say and said should be spelt with different vowels.
Much of Modern English spelling is a result of historic attempts to represent sounds . With printing and dictionaries, spelling became "fossilized" even while the sounds the spelling represented were still changing.
There are some spelling issues that general rules will not resolve.