When did Ma'ariv stop becoming 'optional'?

Question

It's a bit of fun Jewish trivia that Ma'ariv was originally 'optional' (see for example here, and Halachic ramifications of its' original optionality here), but at some point became 'accepted' by Jewish men and is now, for most intents and purposes, mandatory.

I'm curious, when, historically, did the shift from 'optional' to 'mandatory' occur?

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I'm aware that, like many Minhagim, it's probably impossible to pin the date to specific moment, as it likely didn't occur at a specific moment, rather it evolved over time. Realistically, an answer to this question would probably be the earliest written source that says something along the lines of 'it's already accepted among all that Ma'ariv is an obligation'.

Answer 1

The idea that according to the technical law it is a reshut but that it has now been accepted as a chovah goes back at least to R. Isaac Alfasi:

Rif Berachot 27b:

והאידנא נהוג עלמא לשוייה חובה

A century later Rambam already says that this was the custom among all Jews everywhere:

Hilchot Tefilah 1:6

ואין תפלת ערבית חובה כתפלת שחרית ומנחה ואף ע"פ כן נהגו כל ישראל בכל מקומות מושבותיהם להתפלל ערבית וקבלוה עליהם כתפלת חובה

The Evening Prayer is not obligatory, as are the Morning and Minchah Prayers. Nevertheless, the Jewish people, in all the places that they have settled, are accustomed to recite the Evening Prayer and have accepted it upon themselves as an obligatory prayer. (Touger translation)