When I made it, nothing like mermaid, plantUML，lucid charts existed. The syntax of most tools was based on Java or graphviz and overly verbose. I was writing an email to a colleague at BlackBerry and wrote out the sequence diagrams as text. I spent the next week making a script to convert it to an image and put it online. Many other tools have since adopted the syntax and I'm happy it has become so widespread.
It was indeed pioneering in its day, though I find the default mermaid integration in the likes of Notion make it the (generic) goto for the team now.
WebSequenceDiagrams has been my go to for years, and I'd be basically brainstorming straight in it in lieu of pen and paper. I laughed the day one of my colleague was explaining his issue on the whiteboard and he was writing it "A->B: some action" style instead of the actual box and arrow sequence, it just became part of our culture I think.
I had to move on when I couldn't get it approved at the place I worked, and nowadays PlantUML tools have become "good enough" (still wish Notion had PlantUML support instead of mermaid...), but I'm incredible thankful your service is still up and running.