The three dot menu you see on Windows is gone on Linux. Most of the settings available there can also be found in the Settings, but Offline Mode is not one of them.
However, there is a key combination, which can be used to toggle Offline Mode on and off:
Let me know if there's anything else you need help with : )
Help others find this answer and click "Accept as Solution".
If you appreciate my answer, maybe give me a Like.
Note: I'm not a Spotify employee.