That sounds like an impossible problem. Golden retrievers are born-and-bred to retrieve waterfowl and they love water.

what about some sort of fabric mesh retracting gate? Will not work if your dog is determined- but some are stopped by a visual barrier. Or even a cattle panel that you could pull across the opening?

Have you looked into a baby gate? They make some very nice-metal looking ones that even have pass through portions. It's what we've done to keep our Golden Doodle off the pool deck because he launches straight into the water.

Have you looked into a baby gate?

We stayed at a beach house last year that had a big porch across the front ... and a "double baby gate" that matched the railing on the porch. You could swing it open in both directions or close /latch it. I liked it.

If you were to do something like this, you could have the railing extended 3' on each direction ... then have two 3' gates that "meet in the middle".

Or a good carpenter could do some sort of a "sliding baby gate" that could "slide away and stack" to each side.

Invisible gate

Could it be a " barn door" that could slide completely out of the way when necessary?

This may be crazy, but you might put two lightweight matching plant tables on rollers on the left and right of the opening. On those occasions when the door was open you could push the tables together to block the opening. Of course the tables would have to have the underneath have grids or something to keep doggie from running though.

What about a portion of the deck floor that is hinged and you can pull it up...no, that will be too heavy. But what about something along the concept of a top-down-bottom-up shade? Heavy duty but lightweight, maybe vinyl, or aluminum? Unrolls as you pull it up and plop the top bar into two slots on a post at either end? Would probably need a crank to wind it back down properly, though. Or put it on an power control.

Teach the dog(s) to read and put up a sign "do not go down stairs".

(how's that for out of the box)