Which Oxford dictionary did you check?
https://www.oed.com/dictionary/slur_n3?tl=true
1.a. 1609–
A deliberate slight; an expression or suggestion of disparagement or reproof.
1.b. 1662–
A mark, stain, or blot; a discredit (incurred by or cast upon a person, etc.). Const. to or upon.
1.c.1654–
In the phrases to cast, put, throw (etc.) a slur on or upon (a person or thing).
Are you using a different Oxford dictionary? Link us up to the reference please.
or are you just relying on AI?