Opencast has a standing policy of supporting the current version, and the previous version. In general terms, this means a roughly 1 year support cycle for any given release due to our release process (see the development process documentation for more details). Support, in this context, means development time; building fixes, applying them, and releasing those changes. For example, as of the time of writing we currently support versions 3.0, and 2.3. Once version 4.0 releases version 2.3 will no longer be supported, but 3.0 will. This does not mean questions for older releases will not be answered! Our community has a good track record of addressing issues and answering questions regarding older versions as best we can, but developer time will usually not be allocated to fixing additional issues.
In rare cases versions may get extended support, meaning new minor releases are provided for more than two years. For example, the 1.6.x versions have been getting new minor releases for almost two years due to community interest and the backing of some developers and institutions. Such extended support requires community members to fund such an effort, or provide additional support time themselves. If you are in need of extended support we encourage you to reach out to the development community; there may be others in your situation who could share funding, or contractors available to provide this support.