Geofencing has been implemented and will be available in the next version of QField (3.4). The PRs covering the implementation is Implementation of geofencing functionality by nirvn · Pull Request #5409 · opengisch/QField · GitHub . The implementation includes a visual warnings when entering / leaving a geofenced area as well as a digitizing prohibition when in or out of geofenced areas. Pretty exciting functionality! 
Imported from GitHub comment by @nirvn on 2024-08-31T02:28:43Z