The problem here isn't supporting per se the older versions of the OS, but rather the time and effort involved in doing so while maintaining compatibility with the current versions. In all my plugin's case, the minimum is really 6.7.0 (too new for you) across the board, and for the major plugins (CA, FCP) it's 6.9.0+
Decisions to drop compatibility aren't taken lightly, but its a big PITA to even support the Icon for the plugins under 6.6 and still support 6.7+ fully, and every new feature, display change etc all has to be tested down to whatever the plugin says is it's minimum, and there's major differences between how <6.7 and 6.7+ operate for communications, display engines etc. Not to mention the support issues if something isn't working correctly.
Apps - Statistics will let you see everything there.