This is not an official answer, but basically, “support” in this context is more about whether you can ask for help, not whether it will run. Java 1.6 is beyond support but still runs fine in most places. Adobe wants the player to run all existing content. Extensive testing is done before each release to try to ensure that. Adobe in general will only purposefully break content if required to fix a security issue.
And don’t forget, there is continuing development on the Flex 4 code at Apache. If enough folks showed up on the Apache Flex mailing lists wanting to make improvements to Flex 3, such a thing could happen there too. At Apache, development is completely community-driven.