In TYPO3 before v12 the CKEditor v4 was used and it was easy to add buttons and options that came with the editor. Now, with version 12 of TYPO3 and CKEditor v5 everyting seems to be restricted.
The 'full' configuration preset of RTE does not have the buttons to change the text colour anymore and when trying to add the buttons / plugin 'FontColor' myself in a custom JAML configuration, they seem to be removed and nothing happens or an error is thrown.
How can I migrate a TYPO3 project from v11 to v12 where font colours where available?
Actually the features were dropped but are planned to be introduced again.
As far as I see you remarked it already:
As long as it's not yet included the drafted fix can be applied independently. The description on this page helped me to build all files like required to get it running:
How do I activate the font functions in CKEditor 5 in TYPO3 12?
Some hints from me on this page might help even if it's not related to the font features: https://forge.typo3.org/issues/101874
It can probably be expected that the features are included by default again within the next minor releases.