I have an Acer Nitro 5 AN515-55-56R2. The critical low battery warning doesn't show up

Two things can cause a battery won't charge or cannot hold its charge, most commonly is dead cells and that is unrepairable the second reason is the SMART chip inside a Li-Ion battery that is bad or disabled (fire safety trigger) because the battery was depleted to zero. Smart chips can be easily replaced by a battery refurbishing shop and are then sold for <50% of a new battery value, for that reason avoid very cheap laptop batteries as most are refurbished (with incompatible Smart chips). Rules of thumb don't let a battery go far below 50% charge when playing large games as you put unnecessary stress on the power adapter that needs to fast charge the battery and power the system at the same time (overheating) while Windows Power Plan will throttle (reduced performance) to preserve battery power. Replace a battery when the full charged capacity gets below 50% as drainage will become non-linear with unexpected performance (probably your case). Some people wait 8 years before replacing a battery but with the new (OLED) screens, high capacity iGPUs and huge Gen4 SSDs, 4 years is a more realistic battery lifespan, also leave your power adapter plugged in 24/7.

Yes, it still can be charged, try the new power adapter first.