While I appreciate your feedback, it comes back to developing and making the product better as we move forward with version 4 and beyond. It simply is not feasible to continue developing and supporting past versions indefinitely. Discontinuing support is very much a common thing in software development lifecycles as versions age (we actually did kept support for quite some time after version 4 was launched and matured). With version 3, there is nothing stopping you from using an installation, however, you will hit many roadblocks as many of the server infrastructure, 3rd party integrations (such as CAPTCHA), etc... have been discontinued, sun-setted, etc... by their own developers.
Just a note here as we are getting into this information, PHP 5.6, which is the highest version of PHP which can run version 3 of IPB, is no longer supported by PHP themselves for 3+ years now and many hosting providers due to that choose not to run it or have it available to their customers. You may still find some that run it but as it is no longer supported by PHP, running 5.6 is a security concern 🙂 .
@Tschieny, if you have any questions about switching or upgrading to version 4, we're happy to answer any questions. However, if you continue to run version 3 (which is completely fine if you want to), we will move this to our peer community support forum as version 3 is no longer supported.