Why 'shadowban' is hard to check
Reddit doesn't expose shadowban status through any public API — by design, a shadowban is invisible to the shadowbanned user too. What this tool actually does is compare an account's public karma history against what's currently visible in its public post/comment listing. A mismatch (karma without visible content) is a real signal worth investigating, not proof.
Other explanations for low reach
Low engagement is far more often explained by posting in low-activity subreddits, weak titles, or posting outside peak hours than by a shadowban. Check Subreddit Activity Analyzer and Best Time to Post on Reddit before assuming it's your account.