The current situation regarding the
spillover /
overspill pair results from a
discussion in 2018.
I don't intend to reopen the issue, but I can add something to the previous discussion regarding the matter that anona brought up then, of possible regional variations. It seems the use of
overspill is strongly concentrated in Britain and Ireland. In the News on the Web corpus, the word is used 0.26 times per million in Great Britain, but only 0.01 times per million in both the US and Canada. By contrast,
spillover is used somewhat more often in the US than in Britain, but not dramatically so: 0.63 times per million vs 0.35 times per million.
So while the two words are used with similar frequencies in the UK,
overspill is very rarely used in North America.