I could understand that it's a frustrating process waiting for OS versions & browsers to be supported. When it comes to the Slingbox not being able to work immediately with the newer OS versions & browsers, this is how it works.
When a new OS version or browser is developed we have to create new plug-ins for them as well to make them compatible. Once those plug-ins are created the OS versions & browsers are at that time listed as "supported". If any updates come out for the plug-ins you will be prompted to download any updates at that time. Certain OS versions & browsers become "outdated" because we can't keep making plug-in updates for every OS version & browser that is out there.
Below is a link that will give you some aid in installing & removing plug-ins.
I know that this information doesn't fit exactly into what you were wanting to know but I hope it can at least shed a little light about the process of OS versions & browsers becoming supported.
If you have any further questions feel free to reply!
The Sling moderation Team
What shompa is asking for is the opt-in option to bypass the check and try to connect anyways. For example, Gmail will allow a user on an old browser to at least try to use the Gmail web interface even if their browser is not supported. Another option could be that Sling Inc. could create a beta program where users could opt in and provide (useful) feedback and real-world testing against currently-unsupported OS'es and browser revisions.