In Plone 4 we had TinyMCE 3. In Plone 5 we upgraded to TinyMCE 4, which works with a new concept called formats and a new syntax for inline styles, which is
Your Custom Format Title|custom_format_id|custom_icon. For block styles you omit the last part, that is, the icon. As you observed, that's different from Plone 4's
tinymce.xml where you specify
Your Format Title|tag|class.
So after you associate format titles and format ids, you need to define what that format id means. You can do that using the
Formats textarea field. There's already a JSON structure in that field, so you'd add an entry after
discreet, ending with something like:
"classes": "one or-more-css-classes that-will-be-applied"
You can learn all the available options in https://www.tinymce.com/docs/configure/content-formatting/#formatparameters