To be honest I don’t know how long this announcement is on the FSInsider website already, but it seems that MS has run out of copies of FSX. The good news is that they are making new copies, so FSX should be available again at the end of May.
Since it is very important for the health of the FS community that the flight simulation platform can still be bought, I think this is good news. For developers there are still enough challenges to make even nicer addons, so let’s hope this keeps our community as active as usual for some time to come.