It's really hard to say what counts here.
When I first started accessing the Internet, it was the late 1990s and everyone was afraid of Cyber People. Users were encouraged to be secretive about their real names and only give their photographs to trusted people (by e-mail--graphics on websites had to be small or they would never load). Usually, we picked a name and that was who we were to everyone.
That name could be anyone.
I never cooked up a ruse and actively pretended to be someone I was not, but I didn't regularly use my legal name online until my writing career happened. Before that, I mostly used a screenname developed out of a title I created when I was 15, and an associated nickname became the name I actually used in person with people I later met from online.
|Me with some online friends and our screennames airbrushed on our shirts|
When I was in charge of some chat rooms on America Online (an opportunity offered to members in good standing who wanted to get their service for free and were willing to volunteer time), I used to have to use a separate hosting name which was not traceable to my main screenname. In that sense, I did have to play a role, be a grownup, and not talk about myself with full disclosure. But that's a pretty typical course of action when you're in charge of kids. You can't get too personal with them when you're in an authority position.
Years down the line, as in, uh, now, I have done something new online that I haven't really done before. I made a blog on Tumblr--in addition to the blog I already had on Tumblr which is under my long-established pseudonym--and I use it for my fun fandom stuff for cartoons. What's different about it is I don't use my name, I don't disclose much about my identity besides very vague statements of my age, gender, pronouns, and background, and I almost never talk about myself on it. I don't leave any links to my other social media presences and I don't connect them together.
It would be easy for anyone to link them together if they were LOOKING to, since I do post the same art and the same essays in multiple places, but I am not actively trying to avoid "discovery." I just wanted to have a place where nobody really had any expectations of what my content would be like, and to both be honest and sound weird, I wanted to start something where I wouldn't be riding my own coattails; I wanted people to follow me there because they liked what I was making, not because they knew me somewhere else. That way I would know my content was being appreciated for what it was, not because of who I am.
(A lot of who an artist is leaks into what they make, of course, and it reflects on them, but that's another story.)
That's about the closest I've ever had to a secret identity, though, and it's not much of a secret. :P