Thanks for the warning. Did that outdoors. Poked a pinhole in the swollen layer only, let the gasses inside escape and squeeze/flatten the battery case, then covered the pinhole with self adhesive Scotch-like tape. The pressure inside was bigger, so I hope not much air entered the battery. Also it's winter here, so even if some air entered the battery, humidity was low.
I knew abut nasty HF and avoid inhaling, thought it smelled sweet for a short moment. The sweet smell reminded a little about vintage electronics and tube radios, that smell very close to the sweet smell of very old paper books with yellowish pages. Not the same smell, but close.
The battery is a small 300mAh Li-Ion that was so bulged (about 3-4 times its nominal thickness) that I had to remove it from a TEAC mp3 player years ago. It sat bulged like that and unused on a shelf for about 10 years. Charged discharged it before and after deflating. Still working, just that it lost about 1/3 of its capacity over the years. For now I don't plan to reuse that battery.
As for the other Li-Ion batteries leftovers from mobile phones, if I will ever reuse them, will let them bulged as they are. No need to deflate since the DIY enclosures can be made big enough.