I used to use those splices. And I still have the crimper. I got really good at cutting them off w/o damaging the wire using a pair of dykes. but generally speaking they're were really only used in residential on grounds, like in your picture. They can replace wirenuts; you just have to cut the end off even, and there's some 'caps' that you push over the end that snap into place.
I do remember some municipalities hating them, because in their eyes they weren't 'removable' and they preferred being able to be able to untwist wirenuts.
But they were cheap!
Also, in your picture, I don't even think they used the actual crimper for these. A lot of the time people would just take their pliers and smash them in half, lengthwise, around the wires. Which is a shitty way to do it, IMHO.
My guess is, if your house was built in the 90s, that those original electricians were probably the ones to run your GFCI circuits like they did.
There's a theme here, and it's a cheap electrician.
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