There is a “Subtitle” presentation style, but I cannot see how to use it. It seems that “presentation styles” cannot be applied to any text box; for example, you can only use the “Title” presentation style in the “title” object you get when selecting the slide layout. I cannot find anywhere where to add a subtitle.
Related: If someone wants to shed some light on these layout text objects, I dropped some additional questions over here:
You described everything correctly in the reference to your question.
The layout text fields are fields created by LibreOffice and are independent of text fields inserted by the user.
Therefore, use the Drawing Styles for inserted text boxes.
The presentation style “Subtitle” belongs to the lower rectangle in the slide layout “Title and Subtitle”. That layout is the second from left in the first row of the slide layout overview. It is also used for the layout “Centered Text”, the second from left in the second row.
Ooooh, I see! Thank you! I missed it at first, because when selected it says “Text Frame selected”, while for title it says “Title text selected”, and for regular text it is “Text outline”.