The Classic Editor plugin will be supported until December 31, 2021.

The Gutenberg project is a fundamental shift for WordPress. We recognise that there will be a transition period as everyone adjusts to building plugins, themes, and sites using these new tools. To ease the transition for businesses and entrepreneurs, it’s been mentioned that the Classic Editor plugin would be maintained into the future. To give everyone more clarity on how long that’s likely to be, we’d like to add a date to how long the Classic Editor plugin will be supported. The Classic Editor plugin will be officially supported until December 31, 2021.
During that time we will make sure that Gutenberg works seamlessly with existing WordPress infrastructure. That could include tools for testing and updating code, tutorials and additional documentation, or new compatibility layers. What is eventually created will be based on community feedback.
Since the Classic Editor plugin is central in this transition, we are considering including it with upgrades to WordPress 5.0. New WordPress installs would still add it manually, and we’ve included it in the Featured Plugins list to increase visibility. If you have thoughts on this idea, please leave a comment.
Q&A
Source: https://managewp.org/articles/18071/the-classic-editor-plugin-will-be-supported-until-december-31-2021

source https://williechiu40.wordpress.com/2018/11/07/the-classic-editor-plugin-will-be-supported-until-december-31-2021/