TYPO3 Core

Your example is exactly the problem and the restriction ckEditor
is enforcing: You have to prefine sets of classes.
And only these predefined sets editors are able to use.

But richtext-editing mean: Editors should be able to choose
all combinations of allowed css-styles.
rtehtmlarea editors are used to the flexible, modern way of
applying css-class and CUMULATE them.

See my examples above.
Its possible in rtehtmlarea to add a third, fourth ... style.
For ex. a background-style-class, ex "Box-DarkGray",
another fontstyle ex. "LCD-Font" etc. pp.
The status bar would read like this:
p.Text.TextlineBelow.Box-DarkGray.LCD-Font

Most of my editors work on scientific texts.
Applying fonts to super or sub, lines, indents etc pp.
is essential to them. These things are css-prefined
to allow exact editing and proper layout of the text.
I had to remove ckEditor beeing completely unusable for this purpose.

(This is by the way exactly the principle
bootstrap et al. to define their classes.)

With rtehtmlarea your are back-to-the-future in this case.
ckEditor is no option yet at all for this flexible css use.

There is an ticket for this in the ckeditor issue tracker https://dev.ckeditor.com/ticket/11120

Maybe we can find a plugin that provides this functionality

Most of my editors work on scientific texts.

I think that for this specific task, it would be more useful a plugin that helps writing mathematical formulas (I guess that there are several ones for CKEditor)

Thank you for opening a ticket.
I already searched for a plugin for ckEditor and found this hint:
https://stackoverflow.com/questions/29510288/how-to-apply-multiple-styles-to-one-element-in-ckeditor

And there is/was a discussion at ckEditor:
https://dev.ckeditor.com/ticket/11120

PS.
Riccardo mentioned formulas: No, I dont want a specific feature for formulas.
The example above is just an example where the multi-class feature is very handy and usefull.
Adding css-classes is a common and important feature from rtehtmlara
still lacking for some unknown reason in ckEditor for typo3.

As far as I can see there is a commercial plugin for that https://ckeditor.com/cke4/addon/jsplusedittag