R. Natan Slifkin writes:
Halacha says, don’t eat chicken and milk, as a safeguard against eating meat and milk. Judaism requires that we proactively create safeguards to better ensure that we don’t make terrible mistakes.
“Gavriel M” comments:
Halacha says, don’t eat chicken and milk, as a safeguard against eating meat and milk.
There’s not a single source from Hazal that says that. There are certain laws that are identified by Hazal as gezeirahs of this type, though far fewer than generally thought. There are also many cases in which the Bavli alone will declare a particular law to be a gezeirah, sometimes to prevent against something that seems completely implausible. Often, an alternative non-gezeirah-based explanation of that same law is found in the Yerushalmi or other Hazalic sources. However, this isn’t even one of those cases.
Actually, Hazal say exactly what R. Slifkin says, that the prohibition against בשר עוף בחלב is a גזירה intended to forestall בשר (גמור) בחלב:
ואסור להעלות [וכו’]: אמר רב יוסף שמע מינה בשר עוף בחלב דאורייתא דאי סלקא דעתך דרבנן אכילה גופה גזירה ואנן נגזר העלאה אטו אכילה …The mishna teaches further: And it is prohibited to place any meat with cheese on one table. Rav Yosef said: Conclude from this clause that eating the meat of birds cooked in milk is prohibited by Torah law. As, if it enters your mind that the prohibition against eating it applies merely by rabbinic law, this would be because the consumption of the meat of birds cooked in milk is itself a rabbinic decree, lest one come to eat the meat of an animal in milk. And would we decree against placing birds together with cheese on one table due to the possibility of consumption, which is itself a decree? The Sages do not enact one decree to prevent the violation of another decree.
אמר ליה אביי … הכא אי שרית ליה לאסוקי עוף וגבינה אתי לאסוקי בשר וגבינה ומיכל בשר בחלב דאורייתאAbaye said to Rav Yosef: … here, if you permit one to place the meat of birds and cheese on the same table, some might come to place the meat of domesticated animals and cheese on a single table and to eat this meat cooked in milk, thereby transgressing a prohibition
מתקיף לה רב ששת סוף סוף צונן בצונן הוא אמר אביי גזירה שמא יעלה באילפס רותחRav Sheshet objects to the premise of Rav Yosef’s inference: Even if one were to posit that the meat of birds in milk is prohibited by Torah law, ultimately this is still a decree issued due to another decree, as it is a case of cold food in another cold food, consumption of which is itself prohibited by rabbinic law. Abaye said: It is a rabbinic decree, lest one place the meat with cheese in a boiling stewpot, which is a manner of cooking and therefore prohibited by Torah law.
סוף סוף כלי שני הוא וכלי שני אינו מבשל אלא גזירה שמא יעלה באילפס ראשון:
The Gemara counters: Ultimately, even a stewpot is only a secondary vessel, i.e., not the vessel that was on the fire, and as a rule, a secondary vessel does not cook. Rather, one must say that it is a rabbinic decree, lest one place the meat with cheese in a stewpot that is a primary vessel, i.e., that was on the fire. This is certainly cooking meat in milk, and it is prohibited by Torah law.1