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No — Apple doesn't allow any app to read another app's subscription data. Overhead can read a screenshot you take of your own subscriptions screen instead: open Settings → your name → Subscriptions, take a screenshot, and share it with Overhead. It reads the screenshot on your device and fills in the list for you. It's a snapshot, not a live sync, so it's worth re-doing occasionally as things change.
Tap "+" and choose "Add manually" — you can enter any name, price, and billing cycle by hand.
Go to Settings → Data & Backup → Export. This saves everything Overhead knows to a file you can keep or move to a new device, then restore with Import.
Delete an individual item from its detail screen, or delete the app itself to remove everything at once. Overhead has no account and no server copy of your data, so nothing remains after that.
If you add a server or domain, Overhead can look up public registry information (who it's registered to, when it expires) and check whether it's reachable. This only happens for addresses you type in yourself, and only sends that address — see the Privacy Policy for exactly what's sent and where.