By law, sofas must carry a permanent label containing certain information. This is usually found underneath the sofa. There are two versions, long and short. The long version contains the date the sofa was manufactured or imported. The short one does not have to carry this date and the supplier only has to provide it at the request of Trading Standards.
Both long and short must say if the sofa does not contain a Schedule 3 interliner. If it doesn't contain one, it means the cover fabric must pass the match/small flame test. In order to pass this test, most manufacturers have the fabric 'back coated' with a chemical paste that will contain some of the worst flame retardants - the brominated variety. Unfortunately, this paste only needs to last long enough to pass the test (although even then there is often cheating with a properly coated roll sent for testing and under-treated material used on the actual product), which means the stuff often comes off easily (as Trading Standards have discovered).
An interliner can be used with 'cellulosic' materials, e.g. cotton, flax (by 75% or more). This means the cover fabric doesn't need to be coated with flame retardants. Instead, the interliner has to protect the filling material from ignition. Flame retardants are therefore usually used in interliners - in this case, it's the organo-phosphate variety. These were once thought to be less toxic but quite a bit of recent research is showing that they are also pretty nasty too.
So, on balance, you're probably better off with an interliner, but it's not perfect. Until recently, IKEA only sold sofas with interliners. But I heard somewhere that they might have started selling products without, in which case they will probably have to treat cover materials with flame retardants. Check the label.
I don't know where you heard that products made after 1/1/15 don't use flame retardants. But the law has not changed in 30 years so the same tough flammability tests remain in place. It may be that a particular manufacturer has promised not to use FRs after that date. If so, their products are probably going to cost quite a bit more.
Leather sofas in general use less FRs, simply because leather will pass the cover tests without treatment. However, bear in mind the fillings still contain FRs and you might want to try to find out what chemicals are used in making cheaper leather (essentially it's leather dust glued together).
There are some expensive sofas you can buy that claim not to use FRs. This is possible where the cover fabrics are concerned, if say high quality wool is used. But any kind of foam filling will need to be treated with FRs to pass the test. Natural fillings, again such as wool, could pass but that would be even more expensive.