We have all heard about the Lab’s version 1.23 viewer disappearing, no longer available for download. It is no longer just the 1.23. Over the last couple of days several of the older versions were taken off the download server and marked as ‘Not available for download’ in the Old Versions list.
You can find the list in the SL Wiki here.
All versions prior to 3.0.0 have been removed from download availability.
There is information about which viewers can connect to the grid and which cannot. Plus there are instructions for how to get them to connect if you really need to.