Am I going crazy?

Some arduino boards are happy with 10v like Nano Every, I’m powering my with 16.8, the regulator is really good on board

Agreed, but OP was talking of Uno. I think 6.4 to 8.4 is great for an Uno.

Yeah, I was talking about an UNO.

I don't have good experience with the Nano Every.

yeah but is not the reason to throw a perfectly good battery away

Oh I completely agree.
Also Don't throw batteries away ever
There are plenty of recycling facilities.

I didn't feel comfortable keeping it around. It's melted so many wires...

At the time (and honestly even now) I was very new to Arduino so I didn't know what I was doing. If I had the choice now I would have kept it.

I learned that after it was long gone.

Let's get out ahead of the next one. You know not to burn tires?

I kid, but seriously stick around. I'm sure I'll ask the next dumb question and you can ridicule me next time.

I ask ALL the dumb questions.

At least you have a good attitude about it.
Nothing worse than a dumb question and a complete lack of humility to go with it. (Not you, another guy)