Claiming nobody uses DOS anymore is as relevant as me saying I'm not entirely hetero - such information is simply without any meaning.
There is no offcial statement that DOS is not used and even if it was, it would still not matter at all.
Stuff called abandoware is mostly games where copyright holder is hard to find or is confirmed to be out of bussiness. But still, rights may have been transfered to some other entity. E.g. quite a few games from many AW sites (like Abandonia) are not AW anymore, because the now GOG.com has the right to sell them.
One never knows what will be the next game to be excluded, because of new confirmed holder on the scene. You simply never know and you can never be 100% sure this or that game is 'free' to use if it was once commercial title.
ROMs are easier to deal with. None of them are simply free, unless the initial creator says so.
E.g. all Amiga ROMs are illegal to get for free with one exception - if you still own let's say old A500. There is a rather cumbersome, but possible way to transfer them.
If you don't own Amiga anymore, you can legally obtain them only through Cloanto's AMIGA Forever.
Possibly also through some old magazine-included Amiga game packs, which usually also included the ROMs and emulator.
I seriously feel bad when stealing stuff which was hard to produce, especially gems.
Getting 'abandonware' version of Another World e.g. is probably worse than calling Buddha a Christian.
No, 'abandonware' is not FOSS and web 2.0 is full of stuff that is 'free', but of course purely illegal.