“But a Jewish heretic who wrote a Torah scroll—we burn it together with the divine names in it, because he does not believe in the holiness of the Name, and he did not write it for its sake; rather, he thinks of it as just like other things. And since that is his view, the Name was not sanctified. And it is a commandment to burn it, so as not to leave any remembrance for heretics or for their deeds. But if an idolater wrote the Name, we place it in genizah.” These are the words of Maimonides on the matter. It seems from his words that a heretic too is forbidden to write.

It doesn’t say that he is forbidden to write; it says that it does not become sanctified. The matter of burning it is a sanction against heretics, and about that I wrote that it is not relevant nowadays.

Why is there no point today in sanctions against heretics? What is the distinction from the days of the Sages, and since when did the change happen?

Sanctions against heretics are meant to achieve something (to prevent heresy or a respectful attitude toward heretics). Today those goals would not be achieved. Most people are heretics, and the heresy is ideological and not driven by temptation.
I don’t know how to point to exactly when the change happened. But that is the situation today.