Avira,
a good question, I would need to look around; this is a general impression from various mentions in Gemora; Shabbat 15 talks about death penalty ending when sanhedrin left BM, Pnei Yeohusha discusses increase in murderers, prob others also. It may be that secular powers limit beis din, pls review the meforshim there, I’ll try also, bli neder. Re: Alexandria, see Philo
I am sorry to say I am more ignoramus than apikores, I did not learn Frankel. Maybe I his gilgul? I am not sure what I said, but a C-rabbi once asked me “do you believe that halakha changed”. I answered, puzzled “yes”. So, he almost hugged me (that was precovid) and said “so you agree with us”? I answered, with even more puzzled expression on my face: “no”. That was the end of his warmth. Shortest conversation evah.