I don’t know what this means. There is no formal “vow of silence”. The closest things I can think of are the discipline of avoiding unnecessary speech in some monastic communities, or perhaps the seal of confession, but this doesn’t apply as priests can speak in generalities or anonymously about the kinds of moral issues people struggle with.
> Good homilies are written with the particular community in mind.
That’s a bit of a generalization. Many, if not most, readings simply benefit from clear explanation. Tying in local or cultural context can be helpful, but they can also be a distraction, and mostly, homilies should be about the essential meaning of the readings. By having to write the homily, the celebrant benefits from writing the homily as well, a benefit he would lose if he simply drew from a corpus of prewritten homilies.