I haven't seen anything official from SkyTeam yet, but according to this article (SkyTeam Heathrow Lounge is open to passengers of all classes - in Spanish) the LHR/T4 SkyTeam lounge is now accessible to non-ST E+ pax travelling in economy upon payment of a 'small fee' at the door (amount not stated).

While I understand the airlines are keen to maximise their revenue any way possible and, for example, the KLM Crown lounges are already open to some fee-paying pax (only FB Silvers) - I would be concerned if this new policy led to over-crowding of the LHR/T4 lounge for ST E+ and J-class pax. If the Holideck lounge is still there (?) then I don't really see the need for the SkyTeam lounge to take fee-paying pax as well...