Note that watermarking (yes, including text) is a requirement[1] of the EU AI Act, and goes into effect in August 2026, so I suspect we'll see a lot more work in this space in the near future.
[1] Specifically, "...synthetic audio, image, video or text content, shall ensure that the outputs of the AI system are marked in a machine-readable format and detectable as artificially generated or manipulated", see https://artificialintelligenceact.eu/article/50/
[1] Specifically, "...synthetic audio, image, video or text content, shall ensure that the outputs of the AI system are marked in a machine-readable format and detectable as artificially generated or manipulated", see https://artificialintelligenceact.eu/article/50/