If you bought it through Apple, open the App Store app , click your profile name/photo in the bottom-left corner, and find Affinity Photo under your "Purchased" history. Click the cloud icon to redownload.

V1 on Mac - Desktop Questions (macOS and Windows) - Affinity

For the final stable versions of V1 (up to version 1.10.8), Serif provides direct DMG download links for manual installation and updates. Key Version & Compatibility Information

You can still download for Mac if you have previously purchased it. Although Serif has transitioned to V2, they maintain legacy download links and store access for existing V1 license holders. Where to Download Affinity Photo V1