What evidence do we have for iotacism or etacism in this period? Demetrius’
On Style (probably C2nd BCE) shows no consciousness of such coalescence, nor does Dionysius Thrax (C2nd BCE); Dionysius of Halicarnassus (late C1st BCE), on the other hand, describes the vowels as
very distinct forms. None of these sources is κοινη, however.
Still, in terms of later usage,
this summary describes etacism as evidenced in writing (via η/ει interchanges) from the late C2nd CE, and lists individual interchanges (ει/ι, αι/ε, ο/ω, υ/οι) of other vowels rather than the generalised assimilation of most forms to ι.
Also, W. Sidney Allen comments of οι that “diphthongal pronunciation is clearly indicated at least until Roman times” (
Vox Graeca, p.80), and he places ι for οι in the C3rd CE.
While individual versions no doubt varied by regional accent (especially given the non-native aspects of κοινη), Jesus’ own time seems too early for such changes, although a comparison with C1st Hebrew sound forms might be interesting.