Another way on KDE to force the system title bar is by going to ‘Settings → Window Managment → Window Rules’. There you can add a rule (‘Add Rule’ down the bottom) for obsidian. In there you can add the ‘No titlebar and frame’ property, and check the ‘no’ option.
Below is what mine looks like. Depending on your obsidian theme settings you may end up with both the system title bar and obsidian’s title bar. One bonus for KDE users might be that obsidian opens only in the active activity, rather than in all activities.