What is "the LORD’s assembly" that some may not enter?

The chapter doesn't define it. The Hebrew word is qahal. Four of Deuteronomy's other uses are Israel gathered at Horeb: "The LORD spoke these words to all your assembly on the mountain out of the middle of the fire" (5:22), and "in the day of the assembly" (9:10, 10:4, 18:16). The fifth is "all the assembly of Israel" who heard Moses' song (31:30). In Numbers "the LORD’s assembly" names the whole people (Numbers 16:3, 20:4). This chapter names who may not enter it (23:1-3) and who may: "The children of the third generation" of an Edomite or an Egyptian (23:8). Later Scripture uses the word of it again. Lamentations mourns that "the nations have entered into her sanctuary, concerning whom you commanded that they should not enter into your assembly" (Lamentations 1:10). Through Isaiah the LORD says to eunuchs who "hold fast to my covenant", "I will give them in my house and within my walls a memorial and a name better than of sons and of daughters" (Isaiah 56:4-5). Isaiah doesn't use the word assembly or name Deuteronomy 23, and his word for eunuch, saris, isn't one of the words of 23:1. The text doesn't say how these passages fit together.