I guess we'll see much better LLM-based spam filters very soon. LLMs are really good at classification tasks and spam detection would be a great use case for this.
LLMs are incredibly bad at this task and ones like ChatGPT lack the labeled training data necessary to do it. Spam has been classified with ML models at scale for at least 15 years. Spam quality is totally determined by freshness. What you want to know is whether people marked this as spam in the last few seconds, not whether someone labeled similar content as spam in 2019. Finally, the key signals are not present in LLM training sets, since message content is virtually irrelevant to spam classification.