Adobe Flash / Shockwave. After all these decades, I've yet to see a tool that makes it as easy to make games or multimedia as Flash did. One of many reminders recently (many others in politics) that humanity doesn't just inevitably or linearly move forward in any domain, or even 2 steps forward 1 step back. Some things are just lost to time - maybe rediscovered in a century, maybe never.
Even if Adobe had gotten their act together and fixed all security holes, Apple would have still killed it. It was always a threat as a popular design tool. And decades later, with the HTML canvas hype faded, there's still no replacement to what Adobe Flash could do - any designer could create stellar, interactive design that can be embedded into any website...without a monthly subscription.
True, I do think Godot is on the right path, I haven’t had time to look into it in detail, but their HTML5 export seems solid from the videos I saw.