"My senior sister prefers peace and quiet, and does not wish to partake in such lively events, so she remains on Taiyin Star. The Queen Mother will later have the God of Food send her a meal separately."
In years past, there certainly weren't such rules.
But rules, they can be changed by those in power at any time.
Li Changxi indeed does not want to make an appearance at the Flat Peach Assembly, not out of fear, but because she feels her cultivation base has not yet recovered, and she cannot suppress the likes of Empress Nuwa and the Queen Mother, so attending would be pointless.
Furthermore, having killed Daoist Chunyang, attending the banquet in the Queen Mother's territory feels somewhat inappropriate. Though the risk is small, staying home is safer.
Since Li Changxi does not wish to come, Ji Changsheng certainly won't insist.
Which means a bit more work for the God of Food.