ἐπίστασθαι is less common (~ 50 hits) and seldom occurs after the Ptolemaic era.
μὴ εἰδέναι γράμματα occurs 343 times in published documents but only very rarely before the first century CE.
The formulae remain the same: verb of saying (often ppl of φάσκω)/διά + τὸ μὴ εἰδέναι + γράμματα/γράφειν. We can be sure that it's formulaic because the phrase is often abbreviated to, e.g., φαμε μη ειδ γρα, and remains unaltered for centuries. Compare BGU IV 1126, dated to 9 BCE, and BGU XVII 2675, dated to 481 CE.
Verbs in -μι fell out of usage in the common language. Perhaps the same can be said for those in -αμαι? οἶδα resisted for some time because it was so common, the same way the commonest irregular verbs resist normalisation in modern languages. Anyway, I'm sure Gignac or one of those reference works on grammar/historical syntax has an answer.
Statistics: Posted by nate.a — Tue Mar 12, 2024 10:19 am