The WordPress Project says:
The Classic Editor plugin will be officially supported until December 31, 2021.
Since the Classic Editor plugin is central in this transition, we are considering including it with upgrades to WordPress 5.0.
In the context of my professional work with WordPress it would be beneficial to see the Classic Editor plugin be included by default into the installation package. This would signal to plugin and theme developers that we are in a transition phase but supporting the existing installations, until Gutenberg is more fully accepted by the wider community. Not doing do could put pressure on smaller developers to just test their updates for Gutenberg only, resulting in a lot of broken sites or a security nightmare where people won’t be able to update to the latet versions of WordPress or their plugins until the teams and processes are ready.