The OsmTrails.com website does not have any user data, nor have user log in capability.
User data is contained exclusively on a user's device. There is no mechanism built-in to share the same from one device to another device, even where a user has the OsmTrails app installed on multiple devices.
The OsmTrails app uses an on-device database to store GPS coordinates (also testing a second database that includes start stop times to correspond to hiking events). The user can delete all this GPS data by tapping the "RESET TRACKS DB" button on the top left of the main page of the app.
Users have the ability to optionally enter a few emails into a field/setting for use in building a prepared email to forward to an email app on the device. Users can use same interface to delete those emails. The email addresses are not shared in any other way than when you use them to affirmatively send GPX tracks to yourself or others using your selected email.
Users have the ability to optionally enter a few phone numbers into a field/setting for use in building a prepared text message to forward to a messaging app on the device. Users can use same interface to delete those phone numbers. The phone numbers are not shared in any other way than when you use them to affirmatively send a GPS position link text using a user selected messaging app. (Developer uses this feature to prepare for contacting/updating close friends/family.)
User can optionally choose to save cell phone tower bar strengths along with GPS track points. This is used to display them to you, and only you, where you have previously walked. At present the only way to delete this database is to delete the app, and reinstall. If you hike the same route day after day this does not add new data points near those already stored, so as to minimize storage use.
OsmTrails has a page to list tracks that a user has exported from the GPS database on the device. This page allows the user to select and delete these tracks.
OsmTrails has a page to list tracks that a user has imported from others (by way of tapping on an GPX file in the public downloads folder). This page allows the user to select and delete these tracks.
The Android system is designed to delete all of the data items mentioned above by deleting the OsmTrails app itself. This requires no action on OsmTrails.com or by OsmTrails app developer Ron Ledbury.