I don't think the traditional definition of spam really works for bitcoin. Since we don't (generally) have enough information to know why any given transaction was sent, it could just as easily be ignorance or malice that it was with too low a fee. "Spam" generally has an ulterior motive, such as advertising, or installing a trojan on your machine, to get your bank account info, nigerian prince wants to give you money, etc, which is much easier to determine in email or chats just based on the content. But with bitcoin there isn't any content to analyse other than the transaction itself.

In an engineer mindset, it makes more sense to just avoid words like spam altogether, and just label every transaction with a % probability of being included in the next block. Then you just have a useful number, instead of terms which mean different things to different people. Label transactions with numbers, not words, as the blockchain does not care what words we use. Then you can decide yourself if you think everything below a 3% chance is spam, or maybe you think everything below 90% is spam, doesn't matter as long as we have a probability baseline.