Are annotations available on all plans?

Notes on whole articles are available on every plan, Free included. Highlights, and the notes on highlighted passages, need Pro or a higher plan.

Team plans add sharing: a team member can make a highlight or a note visible to the whole team.

Article notes on every plan

Add note below the article and in the article's More menu is there on every plan, so you can write a note on any article and find it again under Saved > Annotations, which is open to everyone too. How do I add a note to an article? shows the steps, and What are annotations? explains how notes and highlights relate.

Highlights on Pro

A highlight marks a passage of the article text, and that passage is what needs Pro. On a Free or Supporter account, Highlight text and Add note in the menu under a text selection open the Annotations upgrade dialog, which reads "Mark key article sections, add notes, and revisit your insights anytime." and offers Upgrade to Pro. Saving a page with selected text from the browser extension needs Pro as well. How do I create a highlight? describes highlighting once you have Pro.

Pro here means Pro and every plan above it, so Team plans include highlights too.

Highlights after a downgrade

Your highlights are kept. The Annotations panel above the article still lists their text and notes, and only the marks in the article text are drawn while the account has Pro. Upgrading again brings the marks back.

Team plans

Members of a team see Highlight for Team in the selection menu and the checkbox Make visible to Team members in the note dialog, which share the highlight or the note with the team.

The Annotations page summarizes it in one line: notes are free on every plan, and highlights are a Pro feature. The Pricing page compares the plans.