I took a look in OED and it has the context of "entitled to a banner as a distinction" first showing up in 1840 with "The Whigs...proposed to designate whichever state should give the Harrison ticket the largest majority, as the banner state"
"Banner year" is cited there first as 1931 but in connection with a race horse breeder, not baseball.
I wouldn't think these are conclusively the earliest uses, but it doesn't look like baseball was the start.
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