At the top of the update notes for Designer 14.5 is this bit:
Ckeditor 4 for Designer/Xpages has been replaced with TinyMCE 6.7. For more information on TinyMCE, see TinyMCE 6 Documentation
My guess is the reason for this change is that CKEditor 4 hit end-of-life in June 2023 and CKEditor 5 dropped the option to license as MPL, meaning the only way to use it in Domino would be to pay for a presumably-onerous enterprise license†. Fair enough.
However, you may notice that the documentation for this change is a little... thin. Specifically, the sentence above is the documentation. Considering that customizing CKEditor has been a popular and long-documented feature of XPages, it's possible you're sitting on some code that used to customize CKEditor to your needs but does not work in 14.5. Since CKEditor no longer ships with Domino, there's no opt-in switch, so we have to deal with it.
Well, I don't have all the answers for how to do this properly, but I did recently have a need to at least start digging into it, so I figured I'd share a preliminary tack to start making cross-version-compatible code.
This is a companion discussion topic for the original entry at https://frostillic.us/blog/posts/2026/2/4/splitting-the-ckeditor-tinymce-difference-in-xpages-on-domino-14-5