The best of the firstfruits of your ground you shall bring into the house of YHWH your God.
You shall not boil a young goat in its mother's milk.
Exodus 23:19

Three times in Torah, seething a kid in its mother’s milk is forbidden in connection with a firstfruits offering. One reason might be in Isaiah 66:3, which equates an unworthy offering with pagan modes of sacrifice, such as pig’s blood and dogs. In other words, even in the least forms of offerings, don’t compromise with pagan religious practices, and don’t mix your offering with resentment or unfaithfulness.

A goat is a clean animal and a valid option for sacrifice. There’s nothing unclean about milk, nor explicitly forbidden about mixing meat and milk. The connection to firstfruits offering, the literary structure of the passage, and the thematic connection to Isaiah 66 all point to this being a reference to a pagan religious practice, probably considered one of the “least” and least offensive of all their sacrifices.