Add availablility

[nichtich]

This will only catch publicly available software, no commercial systems.

  BIND(EXISTS{
    { ?item wdt:P953 ?url } UNION { ?item wdt:P1324 ?url }
  } AS ?available)

[allison-null]

What qualifies as public availability? Depending on how broad you define it (e.g. whether archive.org crawls count or not) some software may either be available or not available (the easiest example that comes to mind for me is the Windows version of MaxThink).

[nichtich]

In it broadest sense availability would be given by an URL to download the software, it's source code and/or a virtual machine with it installed.