In my (nusach ashkenaz) machzor for Yom Kippur, the final blessing after the haftarah of shacharit concludes:
ודברך אמת וקיים לעד. ברוך אתה ה' מלך מוחל וסולח לעוונותינו וכו'
And Your word is true and enduring forever. Blessed are you Hashem, King Who forgives and pardons our iniquities etc.
What is this phrase ודברך אמת וקיים לעד doing here?
It appears many times on Rosh HaShanah (at the conclusion of the fourth blessing of each amidah, at the conclusion of the nighttime kiddush blessing, and at the conclusion of the final haftarah blessing), but this is the only time it appears on Yom Kippur.
If it is as appropriate for Yom Kippur as it is for Rosh HaShanah, then why not include it in the Yom Kippur amidah? Conversely, if it is not appropriate for the Yom Kippur amidah, then why is it appropriate for the blessings after the Yom Kippur haftarah?