Deuteronomy 4:31

לא ירפך means, He will not let loose of thee so as not to hold thee fast by His hands. The expression לא ירפך is an expression signifying: He will not cause something (our Hiphil) — He will not give thee any looseness, i.e. He will not separate you from Him. Similar is (Song 3:4) “I hold him fast, and I will not let him loose (אַרְפֶּנוּ )”, which is not vowelled אֶרְפֶּנוּ (which is Kal and an impossible form of the root רפה “to be loose”). Always the expression “letting slack (רפה)” in the Hiphil refers to one who causes looseness to others or to one who causes it to himself. For example (2 Kings 4:2) הרפה לה means, “give her looseness”; (Deuteronomy 9:14) הרף ממני means, “Give looseness to thyself from off me”.

כִּ֣י אֵ֤ל רַחוּם֙ יְהֹוָ֣ה אֱלֹהֶ֔יךָ לֹ֥א יַרְפְּךָ֖ וְלֹ֣א יַשְׁחִיתֶ֑ךָ וְלֹ֤א יִשְׁכַּח֙ אֶת־בְּרִ֣ית אֲבֹתֶ֔יךָ אֲשֶׁ֥ר נִשְׁבַּ֖ע לָהֶֽם׃

(for the Lord thy God is a merciful God;) he will not forsake thee, nor will he destroy thee, nor forget the covenant of thy fathers which he swore to them.