I am havng a (very rare!) housekeeping session.  I have Cakewalk programme discs going back to Pro Audio 9.  I used to need that one to validate the mp3 codec.  I used to buy disc versions of most updates in case of computer failure.  I have Sonar 5, 6, X1, X3, Dim. Pro. &c. Now we have Command Center is there any point in keeping these?  I know some discontinued plugins may be lurking, but are they any use under Windoes 8.1/10?  What's best practice?  Might I be junking anthing valuable?  Advice gratefully welcomed.

If you have the full Boxed X3 Platinum, I'd be interested in it.  I upgraded from X3 Professional to X3 Producer (digitally through Sweetwater) and hoped to get the install discs from Cake, but they discontinued the discs once they rolled out the Membership system.  
 
 

I would keep the disks that contain the VST and VSTis that you use today but are no included in splat.

Hi. I remember when SONAR shipped with Timeworks plugins (32 bit dxi IIRC, in SONAR XL 2.2), and thought that I would never get the likes of that again. But the plugs included in SONAR today is way better, and I do not miss them at all. I would only keep things used in older projects, and update the rest to whichever is the latest version. Others may have another opinions.
All the best.

Keep everything you have! While we do have CCC it's only for Plat anything before that will not show up in CCC for instance sometime in the 1980's Cakewalk produce a disk called Cakewalk Express while I do not see any need for the plug-ins that it has I do like the demo songs it has those demo songs are no longer available from Cakewalk. Another example Cakewalk had a plug-in Beatscape I think the name is that Cakewalk has discontinue which I think is a very nice plug-in if it was available I would like to have. So again keep everything you have you never know if there was a plug-in that you may find useless that may one day may become useful.

Speaking from recent experience, if you have any old projects created by them, I'd hang on to them.  I've been going through my archives recently and have been trying to open up a bunch of pre-SONAR to SONAR 4 era projects.  They'll open in the latest and audio will work but so many plug-ins have gone away that you can lose a lot of detail.  I managed to find my SONAR 4 install CD and it still works like a charm under Windows 10.  At least now I can look at the old settings as a frame of reference for migrating the projects to the latest if I decide to.

 CCC it's only for Plat anything before that will not show up
 
This is not true as you still have your store account page where everything is still saved. For me this goes back to X1LE. The older programs may not be managed by CCC but if my house blew up and I lost everything I will still have everything I ever bought from Cakewalk stored there and easily retrieved.
I find little reason to keep CD's anymore. Most contain outdated versions of everything including driver instal disks. Now we download direct from the manufacture all we need and it will be up to date. 
 
The exception will be old obsolete equipment. 

If you throw something away you think you'll never need, it is gone, and sure enough, you'll need it. 
 
If you keep something you think you might need in the future, you will never need it....
 
CAVEAT:  Any  saved thing will sit there, year after year collecting dust, tempting you again and again to get rid of the damn thing ... and when you finally give in and toss it ... you'll immediately find you need it.

Some VSTs are no longer available from Cakewalk, as far as I know. Old plugins like the Lexicon Reverb, and V-Vocal, I've had to access by installing them from old disks. For that reason, I'm keeping hold of my CDs.

Agreed with all above especially that nugget from Greg. Keep them and put them nice and neat in a new but nearby location. Label them , put a rubber band around them. Put them in the box that you have that copy of Windows 95, old DOS games and that cookbook software you swore you would use and "Rosetta Stone: Learn Russian Now" and throw THOSE away. Relabel  "Cakewalk Archival DATA" . That way it will "feel" like you cleaned up without actually throwing them away.