In PieFed v1.4 it will be possible to label posts as having AI generated content. Labelled posts will get a little badge near their title, similar to how nsfw and nsfl content is labelled. Mods and authors can change this value on a post in the same way they do flair and nsfw, using the little tags button in the bottom right or in 'More options' when editing their post.
Account settings for blocking:
Similar to how NSFW works, each user can control how those posts are listed on the home page, etc. The default is 'Label as AI', which just adds the badge. People averse to AI-generated content might want to change this to 'Hide completely'.
Also similar to NSFW, entire communities can be assigned as 'AI content' and that will auto-tag every post inside as being AI generated. If the community mods have this value unset (e.g. Lemmy communities, which don't have this functionality) then the instance admin can manually override the community's AI Generated setting.
That seems vague though: when is something synthetic? If I use something like Context Free to create something, would that count? If you paint or take a picture, but apply some filters / effects? It, to me, feels too general to differentiate between tools that "fill in based on training data, and statistics thereof" and those that do not.
While AI/GenAI is used for marketing, it does draw a clear boundary as to what necessitates the annotation, yet is general enough to include any future changes, while being widely recognized.