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Grey area: Discussed in Publish the Android mobile app as FOSS (on F-Droid or as indendent APK) but archived, so creating a new FR on their behalf with clarification
There are some Android devices that don’t have Google Play.
Since Obsidian is free, why not publish the Android apk on F-Droid (preferred) or make the .apk downloadable on the website.
Though Obsidian is not FOSS, it can be published to F-Droid as an official independent apk. This would be preferable because of the integrated update management.