There is an actual rule in place that forbids any sort of obvious advertising, but the rule as it is seems to not be enforced well and/or is simply really easy to dodge.
There has been this trend where artists purposefully upload pictures for the sole purpose of advertising, like a while ago on post #1040974 where I whined like a child about that and the picture actually got deleted for advertising, which got me by surprise (Picture is HERE ). However, there are still picture like these being accepted, mostly YCH bases, where the clear intention of the artist is for people to go bid on their gallery.
I'm not talking about pictures with a signature, watermark or having the URL of the picture being written somewhere, but pictures where the clear intent is presented as an act of advertising alone. There is an obvious difference when advertising is actually done right, like post #1029762 and I'm not talking about cases like these, not even censored pictures, even though I really hate those, but on "art" where the clear intent is to divert the traffic for another purpose.
Good examples would be things like post #748006 post #818606 post #1057295 post #899034 and more recently post #1075736 : The clear purpose for these pictures are for people to be redirected somewhere else as an act of self advertisement.
Now, people could argue that some of them have some art merit, which shows the one big problem about the advertising rule : You can basically snuck in anything you want as long there's something remotely artistic thrown it. Want to advertise your car to be sold on e621 ? Just take a picture of it and draw some quick lineart of a fox telling us to go on his Ebay. Want to advertise your newly made Patreon ? Just throw the P logo in the middle and draw some sketchy headshot of a furry character just below, being sure to write on the description your Patreon URL 5 times because apparently people in the fandom are blind
If you need some examples of what I'm talking about, just go on the FA front page, home of beauties like :
YCH bases [+Reminders]
Characters adopts [+Reminders]
Character selling [+Reminders]
"Stream is now open"
"Open for commissions"
These are the main sort of pictures I'm talking about that e621 should do something on. I just really don't wish for e621 to become FA version 2, where the front page has more in common with Ebay than an actual furry art site
Updated by rysyN