ClassicPress, the community-led project which forks WordPress 4.9.9 has just released a development roadmap covering the first and second versions. As ClassicPress is community-led, the roadmap might change based on the community needs and desires.
It’s extremely important to note that version 1.x of ClassicPress will be fully backwards compatible with WordPress 4.9.x. We won’t introduce any changes or features that would cause plugins or themes to break.
WordPress 4.9 itself will be maintained for the next few years and plugin/theme authors will need to remain compatible with this version. As such ClassicPress confidently state that the vast majority of plugins and themes will continue to work with ClassicPress for many years to come.
The roadmap can be read in full here.