There are a lot of things not specifically prohibited by NEC that aren't legal. NEC is always very clear about requiring LISTED connectors, clamps and other devices. Twisting may be common, but unless we expand 110.14 as carte blanche to use any method we feel like including unlisted knock-off products and home brew bolts, etc, it's not authorized.
I have yet to see anything in NEC authorizing twisting + solder beyond 90.2.(C). Not that 90.2.(C) isn't plenty for most jurisdictions, but it's not the same as actual code allowance.
Not to mention there are other factors involved- the difficulty of reversing such a splice, for instance. Might not be an issue if you've got 3' of slack in a panel as you can just cut it off, but what if every connection in a 2x4 outlet box was made up in this fashion?
