2011? I think? I may be wrong, but it's been a few years.
I don't know when it was last totally closed.
But I believe the way the attraction is built, a lot of maintenance can go on without shutting the whole thing down.
Taking one elevator off line in rotation, etc., - I've been there when one car is down.
The queue spaces are relatively small (lobby, basement) and could be refinished over a period of nights without much notice. They are not enormous halls (square footage) that require a lot of scaffolding or space cleared for lifts. And if you think about it, its not like something "freshened up" is suddenly going to look "shiny and new" and be obvious - it might look DIRTIER lol, but probably not cleaner.
The first-timer, with a few rides over the avg 7-9 days of WDW vacation probably wouldn't notice too much inside.
The outside has been resurfaced a time or two - and that usually involves visible scaffold or equipment, weather proofing tarps etc. But wouldn't necessarily require the attraction to shut down.
This should stop the flickering of the neon lights lol