For me to write "I sixthed it" seemed entirely natural, right up until I tried to to proof read it, aloud.
I'm guessing this is probably where many authors do a double-take and wonder if it is or isn't a real word.
To be fair, "fifthed", "sixthed" and "twelfthed" are particularly hard, each with a cluster of two unvoiced sibilants leaving no vocal resonance for a final unvoiced stop consonant. Others like "seventhed" or "ninthed" are a little easier to say out loud. Whether the following word begins with a vowel also matters.
I suspect we have it backwards when we think these look "archaic": prior to universal literacy, I suspect unpronounceable words would have been, at best, an in-joke for the literate, to be avoided in normal writing. But now when much of our personal communication is written, maybe "sixthed" could become popular before people even notice they can't read it aloud. (And to be fair, it's no harder to pronounce than "???" or "🙼" or "🟔" or "🟠🟡🟢🟣🟤🟥🟦🟧🟨🟩🟪🟫".)
As a practical matter, it's really quite easy to eighth, sixteenth, thirty-second, or even five-hundred-and-twelfth something, even by hand, by the simple process of repeated halving, so that's clearly not the main reason for low adoption for those terms.