I’m not part of the team, but cssclasses are applied to the to both the .markdown-source-view (Source mode and Live Preview) and .markdown-preview-view (Reading view) elements (see Using cssclasses in snippets for a basic intro).
I don’t see those elements in reveal, slides-container, etc.
I have fixed this bug/implemented this feature in a fork of the custom slides plugin (created a couple of weeks ago by somebody else). I can’t link the code so - next time
Now it also supports slide numbers. I will probably try to polish it a bit - I don’t like how the CSS is injected - now that I have the ability to replace the Reveal instance.
I have tried it, but it had other bugs (such as adding a .md extension to embedded .excalidraw files - hence losing their value), that I decided to be worse (and more difficult to fix) than the Slides core plugin.