Backdrop CMS Upgrade from CKEditor 4 to 5 | Backdrop CMS (backdropcms.org)
CKEditor 5 to be Included in 1.27.0 Backdrop plans to include CKEditor 5 in the next core feature version (1.27.0). This will be released January 15, 2024. CKEditor 5 will be enabled by default in all new sites that are installed with version 1.27.0 and higher.