OS X 10.11.6 as server
Here's what I see with 1.7.6 and 1.8.1 beta

1.8.2, mac OS Sierra public beta 5. The icon is still invisible at Synergy startup, but if I click to show the main window from the menu, click on it and clicking back to the icon spot, it become visible but it comes back invisible when closing main window and then clicking on it to show the menu.
I'm using Synergy 1.8.4 with macOS 10.12 (and previously macOS 10.11 w/ Synergy 1.8.2, 1.8.3); for me, the icon stops displaying correctly when "Automatically hide and show the menu bar" is selected from System Preferences -> General.
I never even noticed this issue got solved and I never saw the answer, even if I reported almost 2 year ago.
Are you still seeing this in 1.9?
I tried on a Macbook Pro 2017 with macOS 10.13.5 with that setting enabled ('Automatically hide and show the menu bar') and the icon is still there.
I should test on my iMac (from which I reported and found the issue), but I won't be able for about three weeks. Anyway, if it works on a Macbook pro, it should work even there.
Ok, let me know if this needs to be reopened.
Most helpful comment
I'm using Synergy 1.8.4 with macOS 10.12 (and previously macOS 10.11 w/ Synergy 1.8.2, 1.8.3); for me, the icon stops displaying correctly when "Automatically hide and show the menu bar" is selected from System Preferences -> General.