Lufthansa still has a number of 747-8 and 747-400 in active operation - while there's evidence that the routes are scaling back, there's at least a few more years to fly one. They're even refurbishing the interiors to have a more competitive long-haul business class offering.
Korean Airlines has a handful of 747-8 in active operation but they're making moves to retire them especially post Asiana merger.
Air China also operates a handful of 747-8 and 747-400 on both international and domestic routes.
I flew 747 last month with Lufthansa and asked one of the crew how long they will keep it in operation. «I retire in two years so I don’t care» a very German response but at least they hadn’t made any announcement that he seem to be aware of.
Always fun to be on the second floor despite the seat configuration being a bit dated.
Back in the olden days (2015-ish?) KLM was having a really, really cheap business fare sale JFK-AMS; I snagged it with Delta miles (if I recall correctly) - and flew there and back in their 747 in the upper deck (just to take the flight; didn't have anything to do in AMS). It was really quite nice; it was the first and apparently last time I've taken the 744. I'm really glad I was able to do it.
Somehow I only managed to end up on one of these gorgeous birds once. In seat 64K, NRT-DTW (or was it NRT-MSP?). The main cabin is... nothing to write home about. I was in no hurry to book another 744 leg. Upper deck, perhaps a different story.
No, it is much nicer than the 737/A320 class. Just thinking of the curve of a 737 makes my neck knot up. Bigger planes like the 747/757/767/777 are much more comfortable as well as modern planes like the A220/E195. 737 class planes are so ubiquitous that many passengers have no idea another experience is possible.
The 767 2-3-2 layout is my favorite, with only 1 middle seat per row, yet still two aisles so you can use one while the other is blocked, or walk little loops if it's not.
On the A380 you get to enjoy the higher ceiling also in economy. It does make quite a difference for how cramped you feel, even though the leg room might be the same.
And both B747 and A380 fly much calmer than the smaller, lighter widebodies, which is equally nice for passengers on all classes.
I had a long haul flight from DFW-SYD that had plenty of empty seats to the point they offered an upgrade to guarantee you'd be the only person in the row. Best spent $100 ever related to airfare.
I only ever flew on the upper deck in coach configuration, and the last time I did that was about twenty five years ago on SAA. It wasn't anything special, but it was a little quieter.
I've flown upper deck on a 747 in Business (BA Club World).
It felt like a private jet up there, very cool. And that's even with the awful club world seats where you had to step over your neighbour to get to the aisle.
They are beautiful things, but the last few I rode on with BA were absolutely starting to show their age inside prior to BA retiring them in 2020. I think the last passenger models were produced in 2011 and most of BA's 747 fleet was from the mid-90s. The experience was probably better on other carriers towards the end.
If this is truly on your bucket list, you should be able to pull it off.
I just asked my favorite stochastic parrot to find the cheapest flight from SFO on a 747 to anywhere. It found a one-way flight on Lufthansa for $500. If you can, I'd encourage you to spring for a business class flight on the top deck (probably $4000 one-way).
Korean Airlines has a handful of 747-8 in active operation but they're making moves to retire them especially post Asiana merger.
Air China also operates a handful of 747-8 and 747-400 on both international and domestic routes.
FlightsFrom is a great resource to find routes for specific aircraft: https://www.flightsfrom.com/explorer/FRA?aircrafts=747 https://www.flightsfrom.com/explorer/ICN?aircrafts=747