is it against code to use regular plastic outlet electrical boxes for junction boxes?
NO,they are safer than the metal boxes.
If the old fixture had the cable interior the fixture, then it became and nevertheless is interior of codes. the entire fixture is seen sufficient as an electric powered field. on your new fixture, you may want a connection field to offer appropriate mounting and cord termination.
as long as you use it appropriately, meaning properly fastened, properly secured wires, appropriate length in the box, appropriately sized box for the number of conductors, accessible location, no potential weather infiltration, and properly twisted and capped connections. Oh and a cover. And make sure wires are appropriately coded (white is neutral or colored when used as a current carrying conductor) As for the safer than metal boxes there is nothing unsafe about a metal box, besides you having higher potential of unloading static charge if you touch one after walking across a floor with rubber soles (though that's more of a nuisance than a danger). In actuality a metal box surrounds the conductors with a grounding source to quickly trip the breaker in the event of wire failure (overheating or degradation). But to answer your question simply, it is not against the code to use any appropriately sized, UL approved electrical box as a junction. You can use a nail up single gang, a plastic 1900 with 3 plastic extensions and a plastic 3/4 device ring... just pay attention to box fill limitations. Most plastic boxes you buy today have numbers printed inside indicated how many conductors of 14 or 12 gauge you can have. All grounds count as 1 together. Then you have to count every black, red, white (etc) as 1 each, you have to deduct for internal clamps, (and devices you install) and any wire that is excessively long (12 or more) counts as double. You can't use plastic boxes in commercial applications, or in hazardous environments where damage is likely (such as a shop with machinery) But as for a residential home, and even the garage or basement, plastic is A-Ok.
