Grammarist, "Indexes vs. indices
: posted 2012) viewed on 10 February 2020.
Indexes and indices are both accepted and widely used plurals of the noun index. Both appear throughout the English-speaking world, but indices prevails in varieties of English from outside North America, while indexes is more common in American and Canadian English. Meanwhile, indices is generally preferred in mathematical, financial, and technical contexts, while indexes is relatively common in general usage.
Neither form is wrong. Both have been in English many centuries (and though indexes is now most common in American English, it predates the United States by centuries). It's true that indices is the plural of index in Latin, but index is an English word when English speakers use it-and it is a longstanding one at that-so we can pluralize it according to the conventions of English.