Thank you all for clarification. Lesson learned. Maybe a little late, but we don't know what we don't know.
Next question would be if anybody experimented with puncturing the skin of the battery with a small hole (without touching the internals) in order to deflate it, then covering the small hole with some adhesive tape to seal it against air.
I've seen videos of people doing that (outside, toxic gases and so on), and read online about how they continue to use the batteries for years after deflating them. Others are saying aged Li-Ion batteries are a fire hazard and to never use them once they bulge, and usually the former will link to a few "classic" videos with intentionally ignited batteries.
I've never seen or heard of igniting Li-Ion in the circle of known people, yet everybody I know uses phones, tablets, laptops, cameras and so on.

I would like to keep using the bulged batteries for small current circuits, weather station, hallway LED lights with moving sensors, things like that where the low self discharge rate of Li-Ion will make them last much longer than the NiMH cells.
Anybody tried to deflate the Li-Ion then keep using them?