It's a common term.

There's also "x"/"cs" -> "xen"/"csen" (a la "ox" -> "oxen"). See: Emacs -> Emacsen

Like indices vs indexes, either of which is generally accepted as valid.

Yep! Same for linux -> linuces.

The Latinate -ix suffix becomes -ice/s when pluralized, as in matrix and matrices, or dominatrix and dominatrices.

However, -es (as in "unixes") should be perfectly acceptable too. Just like how "indixes" and "matrixes" are fine in English.

I've seen "indexes" and "indices", but never "indixes". I assume that's a typo. Or is "indix" a noun I don't know?

Yeah, that's a typo. Sorry about that.

Oh? Thanks for the information, I love to learn about English, it has so many quirks!

But...UNIX is not Latin, but seems to be a corruption of an English acronym. So a pedant may argue that this is incorrect.

“[A]lmost anything ending in ‘x’ may form plurals in ‘-xen’ (see VAXen and boxen in the main text). Even words ending in phonetic /k/ alone are sometimes treated this way; e.g., ‘soxen’ for a bunch of socks. Other funny plurals are the Hebrew-style ‘frobbotzim’ for the plural of ‘frobbozz’ (see frobnitz) and ‘Unices’ and ‘Twenices’ (rather than ‘Unixes’ and ‘Twenexes’; see Unix, TWENEX in main text). [...] The pattern here [...] is generalization of an inflectional rule that in English is either an import or a fossil (such as the Hebrew plural ending ‘-im’, or the Anglo-Saxon plural suffix ‘-en’) to cases where it isn't normally considered to apply. This [...] is grammatical creativity, a form of playfulness.”

http://catb.org/jargon/html/overgeneralization.html

People do it anyway for giggle value.

I was using "Pentia" as the plural for "Pentium" soon after the original chip dropped.