⭐ Merited by ETFbitcoin (4)
It is not so. At least about 15 years ago I already used to get a reward for placing advertisement in my signature on some forums. It was a time when SEO links were very actual, so many were very motivated in advertising via pass-through links. It was different, because the amount of already left posts mattered and it was less important how active you are at the moment. But well, I was paid for advertisement in my signature. Of course there was no BTC at that moment, so we got the reward in USD and much less than projects pay for it at the moment. But I first found it much earlier than came on bitcointalk.org and even earlier than bitcointalk.org first appeared. 
Do you remember name of forum where people offer payment for placing signature then create post? Such forum probably dead, but i want to did quick research for my own curiosity. I tried using search engine, but it shows either Bitcointalk or other forum which pay for posting (without involving signature or other user space to enter text/bbcode).
I can give you even a link to a web archived section of one forum where there are several topics about buying ad places in signatures on different forums (you could make a deal there on selling an ad place in your signature on many other forums, if it was allowed on those forums). It is in Russian. Topics with a word подписи/подписей in the title are about that.