Overly superlative adjectives and too many adjectives in general. 25 years ago, well before LLMs were a thing, I was a tutor at my university's writing center teaching people to strip this from their own writing. It adds time and effort to the process of reading without adding information.

There's room for lyricism and artistry in poetry or other forms of writing meant to entertain or evoke particular feelings, but not in a thesis. A thesis should get to the point using the path of least action. It's the difference between calling something a "many-sided pseudopolygon with no vertices halfway in tint between blue and red" and calling something a "purple circle."