My regular steamdeck died on me a while back, did not charge anymore and did not power on. Was well out of warranty, messaged Valve, RMA'd it and got the repaired unit back. Zero cost. I'd just contact support and see what they say if you haven't already.

Try sticking it into battery storage mode for about 30mins then restarting it using the mains lead

https://www.ifixit.com/Guide/How+to+Enable+Steam+Deck+Battery+Storage+Mode/149962

That process won't harm the steam deck and resets a lot of things if they start acting weird, like the touch screen as well.

Have you measured voltage across the terminals?

Try recalibrating the battery

Multimeter, as another poster said. Be careful not to short the terminals. I do recommend you ask someone for guidance who have some electronics experience, lithium batteries can be a little dangerous if you are very inexperienced... And even if you have some experience.