Most PDF viewers support adding text in expandable sticky notes, highlights and strike-trough, etc. I know typst supports highlights, strike-trough, and many other features already. I also know comment bubbles can be added with e.g. the drafting package.
What differentiates “PDF annotations” from existing typst functionality mentioned above is that they can be shown/hidden and edited in many PDF viewers. Those PDF annotations are portable too, in the sense that they are embedded in the PDF document itself as an additional layer. They are also portable across PDF viewers (I.e. annotations made in one PDF viewer can viewed/edited in another, something I tested with e.g. Zotero → Evince).
For example in the image above, the transparent black box with comments only appears after hovering over the sticky note. In this case I’m viewing a PDF in evince, while the comments and highlight were made with Zotero.
It would be nice if typst supported adding those from typst source code. Especially for reviewer/collaborator comments. Packages like drafting are nice already but because reviewer comments are part of the document, they also have an effect on the final layout. Hence, you need to toggle a boolean flag on/off before compilation to evaluate what the document would look like without comments.