tip: "sherlock (tv)" m/m NOT "sherlock holmes/john watson"
Seriously. Most sites have a small banner at the top of the page that you can acknowledge or ignore. The idea that AO3 is at particular risk and so has to block their entire site with a message is kind of ridiculous. The GDPR is targeted at sites like Google and Facebook. The EU isn't going to come knocking at AO3's door, a site which collects virtually no user data, comparatively, because they failed to be as obnoxious as possible to every visitor.