On macOS 26 (Tahoe), with DT 4.9.15, the “Style” button on the Format Bar for Rich Text files seems to have disappeared. (The Style button is the left-most button in the Format Bar, which has a paragraph glyph.)
Interestingly, this is also the case for macOS TextEdit, but in TextEdit the button DOES reappear if you make the TextEdit window wide enough. However, no resizing of windows in DT seems to make the Style button re-appear. The Format Bar in both apps seems to have undergone a refresh in macOS 26, so perhaps there is some incompatibility here (or I’m missing something obvious in a menu someplace).