Is edit history available in all plans?

Every plan shows the Edited mark on articles the publisher changed. Opening the edits, with the list, the changes shown in the article and the earlier versions, is part of Pro, Team and Custom plans.

The preview on Free and Supporter

On Free and Supporter accounts the mark reads Edited with the time of the last edit and View edits with a PRO badge. Clicking it opens a Pro dialog with a preview of the latest change: a card with the feed name, the time of the edit and the first changed lines, with added words in green and removed words in red. The preview is real for three articles a week, counted from Monday to Sunday, and opening one of those articles again keeps its preview. For further articles that week the card is blurred and reads Upgrade to see what changed. The dialog offers Start a free trial when the account can still start one, and Upgrade to Pro otherwise.

Supporter doesn't include edit history, so a Supporter account sees the same preview as a Free account.

The full history on Pro, Team and Custom

Pro, Team and Custom plans open every edit of every article: the list of edits, each change shown in the article and the versions from the one as first published, up to the 20 edits Inoreader keeps per article. How do I see what changed in an article? walks through the list and the changes, and what does "Edited" on an article mean explains which changes count as an edit.

Edit history after an upgrade

Inoreader records the versions of every feed whatever its readers' plans, so when you upgrade, the history of the articles already in your account is there, with no setting to turn on.

The pricing page lists article edit history under Pro, and the feature page See what changed after an article was published gives the overview. Annotations are gated in a similar way, as are annotations available on all plans explains.