I think 5,703,999 is about shifting between using joint and separated stereo for frames depending on the amount of difference between the channels to avoid cross-channel interference, this sounds like it might still be important for lower bitrate mp3s.
I don't think I know the technical details well enough to comment on 5,924,060, but I'll note that it has a priority date of 1986 and publication date of 1999... good riddance to submarine patents.
So the Tunequest[0] article referenced got it wrong on the last two? I have a hard time figuring out what applies and does not apply to patents filed between 1995 and 1999
See: https://madfileformatscience.garymcgath.com/2016/0/05/mp3pat...
The last two patents seem to be related to an encode optimization and are not needed to write a decoder.
Which means it is possible now to include an MP3 decoder in GNU/Linux distros, etc. without paying the $2,500 fee.