index はラテン語由来なので複数形はラテン語系の屈折と -es をつける英語由来の屈折とに分かれます。ODEでは plural indexes or especially in technical use indices, CamGEL (p. 1592) では With index the two plurals usually correspond to different senses, indexes applying to alphabetical reference lists in publications, indices to raised numerals in mathematics. とされている通りです。OED では次のように書かれています。
In current use the plural is indices in senses 8, 9, and usually in other senses except 5, in which indexes is usual.
5. a. A table of contents prefixed to a book, a brief list or summary of the matters treated in it, an argument; also, a preface, prologue. Obs. b. An alphabetical list, placed (usually) at the end of a book, of the names, subjects, etc. occurring in it, with indication of the places in which they occur. One work may have several indexes, e.g. an index of names of persons and places, of subjects, of words, etc. For these the Latin phrases index nominum, locorum, rerum, verborum are often employed as headings. c. A reference list. Obs. d. Computers. A set of items each of which specifies one of the records of a file and contains information about its address.
8. Math.
9. In various sciences, a number or formula expressing some property, form, ratio, etc. of the thing in question.
要は「現在は indices を〔数〕あるいは科学的な指標として使い、他の意味でもおおよそ indices である。ただし、索引という意味でだけ indexes が普通である」ということです。
