'Data' is attested as a singular mass noun since at least the early eighteenth century - (1702 R. Morden Introd. Astron. i. 103 And by this Data there are twelve Problems resolved." (OED). Indeed in the 17th century 'datas' was an acceptable plural (1645 T. Urquhart Trissotetras 53 The verticall Angles, according to the diversity of the three Cases being by the foresaid Datas thus obtained) which shows that the singular form was understood to be 'data'. For the lexiographer, this means that datum and data are effectively two different words with their own grammatical properties, which is why they have two separate entries in the OED. Let it go.