We supported our extensions on Joomla 1.5, 1.6 and 1.7 well past the official end of life of those versions. But Joomla continues to evolve rapidly, and it became impossible to prepare our code for future versions while maintaining support for the obsolete versions. Each new release of Joomla removes older functions, replacing them with new ways of doing things.
In the latest releases of our products, we removed the code that supported Joomla 1.5, 1.6 and 1.7. That allowed us to simplify the code, improve our support of Joomla 3, and prepare for future versions of Joomla by migrating deprecated functions. Major new versions of Joomla always seem to have a few surprises that break our code, but we are actively preparing for future releases as best we can.
We will continue to support Joomla 2.5 until at least its official end of life - December 31st, 2014. There's a handy table of Joomla versions here
We will continue to make the legacy versions of our products available for download until the end of 2014, but these old versions will no longer be developed or receive bug fixes. We will always do our best to help you if you have a problem with one of our extensions whatever your environment, but our memory of Joomla prior to 2.5 is fading fast and if you encounter a tricky problem, we will probably ask you to upgrade to a current version of Joomla and the latest version of our product.
If your site is stable and working well, there is no need to upgrade it. But the more out of date a site gets, the harder and riskier it is to upgrade. You can put the job off for a while, but eventually something will break and you will have to upgrade. We are now finding that it's actually easier and safer to keep up with Joomla upgrades as they happen.