End of Engineering support means no new major versions of software for that hardware.
Existing versions at the time of End of Engineering support will still be supported until End of Life.
After End of Engineering support, fixes specific to that hardware will be limited and may require software and/or hardware upgrades.
To give a concrete example, the appliances we released in 2016 have an End of Engineering date of end of 2023.
At the very least, this means R81.20 (latest release at present) will be supported on these appliances until their End of Life.
R82 (next major release) may or may not be supported on the 2016 Appliances, depending on when it is released.