The sporting covenant on Oakwell will run out? I'm sure that in JD's book he intimated that it was upon the death of the final grand child of those who gave the land to the club in the first place. Can't be far off now?
Given that Oakwell has changed hands more than once in recent times, I would imagine that it must be registered land. As such, any restrictive covenant would have to be recorded on the charges register, I believe (although I'm not a property lawyer). The register is open, so anyone sufficiently interested could get official copies of the title register from HM Land Registry at Nottingham for a very modest cost. In more general terms, both the benefit and the burden of the covenant has to run in order for it to subsist. As the original donors of the land are unlikely to have an ongoing personal interest in it, I doubt whether any such covenant would still be enforceable. In practice though, where there is doubt about such matters, common practice is to obtain indemnity insurance against any future enforcement of a covenant if you want to do something that might technically breach it.
I imagine that would leave them in the position of a) not being within the parcel of land to which the alleged restrictive covenant attaches or b) are with in it, but the purchasers or, probably more importantly, their mortgage lenders, have taken the view that the covenant is not enforceable or they have taken out indemnity insurance to cover against it being enforced.
The first time I heard of this covenant was way back in the early 80's and if I remember correctly it only relates to the actual pitch/ ground area but that info was only 2nd 3rd hand.
The Co-op Sports Club, the Oakwell Brewery and Grove Street School are all now defunct and the Council is joint owner of the ground. So I can't see who could enforce such a covenant. I don't think nearby residents could object to more housing. I don't think therefore there could be any objection to PC/Family selling the land for housing and using part of the proceeds to build a cheaper stadium on non-residential land (e.g. Dod'eth). Other than the uproar from the fans, that is.......!
I can't see a property developer seeing the Oakwell area as a big draw anyway, there's plenty of land near places with better road links.