Before the bio, there was the forum signature
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In 2008, I got my first laptop. I was obsessed with GTA San Andreas and wanted to play it online, so I downloaded the multiplayer client. Nobody made me register on the forums but it was the obvious next step. That's where the servers were listed, where mods were shared. To some extent, where part of the game lived. I was new to all of it and I spent equal parts time reading threads and playing. At some point I started noticing what appeared below every post: a little strip of personalized space, different for everyone. Mine was blank. Other people's were something else entirely. 400×100 pixel GIFs of characters from whatever game they were obsessed with that month, rendered in the kind of neon gradient that could only have been made in Paint.NET by someone who had just discovered the smudge tool.

"My gaming forum signatures from 15 or 16 years ago", by Reddit user theskyguyuk.
That was the signature. A tiny patch of internet real estate, appended to every post you ever made, that said: this is who I am, or at least, this is how I want to be seen in this community.
The modern equivalent is the bio. That 160-character X field, the "About me" blurb on LinkedIn, the Linktree in an Instagram profile: those are functional. They're optimized for discoverability, for the algorithm, for first impressions from strangers who will make a judgment in under two seconds.
Forum signatures were none of those things. They were decorative, personal and under no pressure to gather an audience.
You'd spend more time crafting your signature than writing your 10 latest posts. There was a whole grammar to it. Maybe a quote, either something profound from a philosopher you'd half-understood or something deliberately ironic. Maybe a troll face. Maybe a little text badge declaring your allegiances: Linux user. Photoshop user. Firefox, not IE. Small signals to the right people that you were one of them.

"Anyone remember the forum sig craze about 15 years ago? This is a collage of some of mine, happy to have kept them for memory's sake. Simpler times.", by Reddit user SometimesGreydn.
There was a whole ecosystem around this. Dedicated threads for "signature shops", where people with slightly more skill in Photoshop would make graphics for others, for free or for forum reputation points1.
The signature also had a strange property: it was retroactive. Every post you'd ever made now wore your current one. Change it today and you'd rewritten yourself across years of history.
I'm not trying to be nostalgic about forums in the way people are nostalgic about vinyl or film cameras, i.e. the "worse but more authentic" argument. Forums had plenty wrong with them. But the signature was genuinely a different kind of self-presentation. Nobody was personal-branding their way to a sponsorship deal through their forum signature2.
Forums died3 when social media made them feel like too much work: too many URLs to remember, too many accounts to maintain, too much friction compared to a feed that just existed on your phone. Why maintain an account on twelve different community sites when Facebook Groups, and then Reddit, and then Discord, could approximate the same thing in one place? The audiences migrated and signatures went with them. Reddit never really had them4. Twitter/X never did.
Maybe that's fine. Maybe the signature was always just a toy. But I do think something was lost when identity online became something you optimized rather than something you decorated.

"Forum Signatures", by unknown Reddit user.
Footnotes
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A number that went up when people liked what you said. ↩
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Well, maybe not nobody. You'd typically see a link to a Youtube channel or something, but it was never the point of the signature. It was more of a "hey, if you like what I say here, maybe check out my Youtube". ↩
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I say "died" but of course they didn't really die. They're still there and some are still active, but they're no longer the default place for online communities to gather. ↩
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Reddit has "flair", which is a little badge that appears next to your username, but it's not really the same thing. It's more of a status symbol within the subreddit, rather than a personal signature that appears on every post you make across the site. ↩