Rich text in the modern world is almost exclusively solved by using markdown because it’s such a trivial solution.

citation needed

markdown is not a trivial solution: there are many different implementations, it’s a barrier for non technical people and it allows you to embed any html, so you need an additional html sanitizer.

my definition of a “rich textbox” is a WYSIWYG field, and markdown does not help you with this?!

yes, you probably would not save the formatted text normalized over multiple database columns, and only use a single field for a the text with formatting embedded in html or another format, and another one with the text without formatting for possible full text search. but even if you would solve this using markdown (which limits you to a quite small subset of text formatting and bad extensibility) you would still need a good data format to store the formatted text in memory that allows you to render the text. and markdown does not help you with this either?!

hoped to discover a good open source software for collaborative editing, but their solution is based on https://standardnotes.com/