Siman 609
Presented here for study, not as a practical ruling. The Shulchan Aruch rules by Sephardi custom, the Rema's glosses by Ashkenazi custom, and the Kitzur follows Ashkenazi custom. For a practical question, ask your rabbi.
א מֻתָּר לְהַטְמִין חַמִּין מֵעֶרֶב יוֹם הַכִּפּוּרִים לְמוֹצָאֵי יוֹם הַכִּפּוּרִים. הַגָּה: וְיֵשׁ אוֹמְרִים שֶׁאֵין לְהַטְמִין בְּיוֹם הַכִּפּוּרִים (עַיִּןִ בְּטוּר), וְכֵן הַמִּנְהָג בִּמְדִינוֹת אֵלּוּ (מַהֲרִי"ל).
1 Covering hot food from the eve of Yom Kippur and in it is one paragraph. It is allowed to cover hot food (i.e. to maintain warmth) from the eve of Yom Kippur for Motzaei Yom Kippur. Rema: Yet there are those that say not to cover on Yom Kippur [see in the Tur] and so is the custom in these countries [Maharil].