Whew, that's a loaded one. Lemon as a term shifts depending on fandom and era, honestly. Back in the early 2000s on places like Fanfiction.net, it was a straightforward content warning—explicit sexual content. Now, with AO3's extensive tagging system, 'lemon' feels almost nostalgic, a relic from a time before 'Explicit' and 'Graphic Depictions of Sex' were standard tags. The definition being fuzzy can cause issues; some writers use it for mild explicitness, others for hardcore stuff. That ambiguity means readers might stumble into something way more intense than they bargained for, which is why clear tagging is so crucial now.
I think it affects guidelines less about outright banning and more about categorization and filtering. Modern platforms rely on the creator to accurately tag, so a story tagged 'lemon' but also 'fluff' and 'romance' sets a different expectation than one tagged 'lemon' alongside 'dark' and 'non-con'. The old-school definition forced a binary: is it lemon or not? Today's approach asks what kind of explicit content it is, which lets communities build more nuanced guidelines around consent themes, kink, and violence. It's a move from simple age-gating to actually describing the content, which I vastly prefer.