The display is plasma not EL. Same was used on many 80's era laptops and luggables like the Toshiba 3100. They can suffer burn in from static displays just like CRTs and modern plasma screens.
https://en.wikipedia.org/wiki/Grid_Compass
https://history-computer.com/ModernComputer/Personal/Grid.ht...
Everywhere says it's electroluminescent. I mentioned "burning out" instead of burning in. I know it's not the most technical term, but I mean LEDs failing in displays. In your speedometer for your car, either the whole display with fail (probably the circuit is broken) or the whole thing works, because it's one EL panel behind a translucent screen.
Also because of the nature of an ELD's components being static, (they radiate when a charge is applied, in a way very different to the phosphors that move around in CRT and plasma screens) I don't think it's possible for them to burn in.
And notwithstanding that, they anyway weighted some 5 ot 6 Kg (but they were tough and had a "proper" keyboard).
It is not clear if the Grid Compass was EL or plasma, here: http://www.old-computers.com/museum/computer.asp?c=900 they talk of amber plasma and surely the photo here:
https://history-computer.com/ModernComputer/Personal/Grid.ht...
https://history-computer.com/ModernComputer/Personal/images/...
looks a lot like amber plasma (as the Toshiba 3100 and other "portables" of around that time).