Word has had that for a long time. In modern ribbon-interface Word versions, it's in the Home-Paragraph section, top right icon ("paragraph mark"). Additionally you can/should toggle field code display with Alt-F9.
This has been in Word since, I think, Word for Windows 2.0 (circa 1992, for Windows 2.11). It might have also been in the previous DOS-based Word versions that co-existed with DOS-based WP (but since I used both, I can't always tell which features were in which).
Maybe not markup language, but the idea is to show the markup. To get a paragraph break, you still hit Return and don't try to type that end-of-paragraph marker.
The field codes are more like an actual language; you can insert a field and then type the content in it.
This has been in Word since, I think, Word for Windows 2.0 (circa 1992, for Windows 2.11). It might have also been in the previous DOS-based Word versions that co-existed with DOS-based WP (but since I used both, I can't always tell which features were in which).