What Happens If You Max Level

Do you still get key points from the planes?

The key points only max when you have no further slots to open.
The levels part of the game does not affect this.

So if you finish the INN levels. You will continue to get points on your open destinations when aircraft depart until every is completed.

In the example you posted. They will keep going until you have opened all 10 arrival slots and 5 departure slots.

The golden keys are only picked up by level completions.

Hope that answers your question

Oh ok

Do you mean when you unlock all possible slots for this destination, 10 arrivals and 5 departures?

Then the meter will fill up one last time but do nothing, and after that you’ll stop earning keys for that destination.

No I beleive he means max airport level