There will certainly be less (perhaps none), feature back-ports from stable releases. (Some Unity features from the regular release are sometimes backported to the last LTS)
Unity itself doesn't appear to have any past CVEs, but if there is one I am confident it will be fixed.
The biggest issue I can see would be apps removing/ending support for the Unity GlobalMenu - or the patches becoming to difficult to maintain for Firefox version 70 something.
I'm not saying I want people to consider staying on Unity7 until 2021, just that you can and still get updates. I'd prefer if everyone used the most recent LTS myself.