CKEditor - Real-time collaborative future-ready rich text editor
Sitecake - Drag and drop CMS for HTML websites. It's flat file CMS so it's pretty fast.
TinyMCE - TinyMCE is a content editor that functions as a plug-in for Wordpress websites.
ClassicPress - The WordPress fork. No Gutenberg. Great future!
Quill - Powerful, API-driven rich text editor
TYPO3 - TYPO3.com - Infos, SLAs, Extended Support Versions and more