Is it acceptable to travel in shorts and sandals?

I opt for loose palazzo-type pants which are v comfy but look put together enough. I HATE the texture of BA’s faux leather seats (and the thought of other bums on it) so shorts would be an absolute no.

btw - as a group there is next to no chance of upgrade - as a single or couple perhaps.

I would say wear whatever is comfortable to travel in and is suitable for your destination. I travelled solo to some mid eastern countries including Iran so I followed the dress code.I would not want to arrive in a Moslem country wearing shorts and a skimpy top. Long. loose and coverup.

Personally I always wear enclosed shoes and socks when flying. I don’t know why, but it feels a bit gross to wear flip flops/sandals.

Oh no, no, no 😆You couldn't be more wrong. I regularly travel via Mykonos and that image that you have is way off track! It HAD a reputation for being a chic & glam destination but truly lost that way before 2020. These days it's mostly overpriced, overcrowded & awash with Love Island wannabes taking endless selfies🙄

OP wear whatever you feel comfortable in for your flight. There is no special "dress code" for Mykonos flights and as others have pointed out, an upgrade is extremely unlikely and will not be influenced by your outfits. Nor are you more or less likely to be requested to put your carry on in the hold. BTW, if you did put your bags in the hold, Mykonos Airport is very small and it takes very little time to retrieve your luggage on arrival. The longest wait is usually for passport control. In my experience, people generally dress for comfort - often loose fitting trousers or shorts with t-shirts. I often wear Birkenstocks for travel because they're comfy & easy to slip on & off if required in security.

Tbh when I fly I’m just freaking out about the plane going down so I just wear whatever is going to make me sweat the least.

I always dress for cooler weather, regardless, because 1) planes are cold and 2) you need something for coming back to the chilly UK.
I do think sandals are inappropriate..... I don't want to see other people's feet on a plane, plus a grubby airport is not a place where I want to expose my feet.
And I'm old, so I don't wear shorts in public (only on a beach, or by a hotel pool, and even that is pushing it 😂).
Long floaty trousers are much more elegant, and equally cool
But the answer to the OP's question is that everyone can dress however they like - why do they all have to look the same?

I once had a debate with my husband about being upgrading. He and I both have dietary requirements that we declare beforehand. I said ruefully when about to go long-haul 'We have lost our chance to be upgraded then' and he said he felt sure that having a special meal did not eman we would not be upgraded, but that our ordered meal might be delivered to us in Business. I feel sure that's just way too much faff for the airline tbh!

In any case, when we travel we wear comfortable clothes- jersey harem pants and long sleeved tshirts for me. But that's mainly because we go to Australia or New Zealand and it has to get us through.

It doesn't make any difference what you wear.
I've been upgraded twice. Once by an airline where I was part of the loyalty scheme, long time ago now.
And once last year, not part of the loyalty scheme but at the gate they upgraded me and said it was because they wanted to sit a late checking in family together.
Both times I was in my usual scruffy hiking stuff about to start a long-distance hike.

Where are you travelling to? Tgat us key to what you wear.

I think the man-made fibres advice is mainly applicable to the light aircraft/small aircraft world but even in the airline enviroment it's not utterly daft advice

There's a problem here in that the airline industry pushes how safe scheduled airline flying is, which it really really is, and so any safety advice at all tends to get blown off...When I was working it was hard enough to get some people to listen to the safety briefing.

I doubt many are going to take advice on choice of fibres based on safety grounds but having known and talked to a handful of people (crew) who have been evolved in Evacs I,'d still recommend not flying in anything that isn't going to stay attached to the feet....

Even clean nicely pedicured feet?

I wouldn’t wear sandals, personally - too much risk of injuring toes in an airport and airplane environment with all that luggage about and clumsy people in a rush. I also wouldn’t wear shorts, because I would definitely get too cold on the plane that way.

Just got off a flight, considered wearing shorts as it was hot leaving the UK and knew it would be hot arriving in Finland but glad I opted for trousers (scruffy hiking trousers because useful pockets and lightweight) because it was cold on the plane. Changed into shorts in the loo when I arrived though.
Zero chance of an upgrade anyway, I was on Ryanair

or at all since the 1950's?

As someone who has spent their entire 20 year career handling claims made by passengers injured in crashes and evacuations I can confidently confirm that I have never once handled a file involving injuries that were exacerbated by man-made fibres.

This is my thinking too. I would just feel a bit vulnerable in sandals.

I only ever fly EasyJet so upgrades aren't really an issue.

Also….. it can be cold on planes. I once wore shorts when travelling at this time of year to the Caribbean. I was cold. Normally I wear lightweight jogging bottoms, a t-shirt and thin long-sleeved top.

Shorts are fine, however I only wear trainers or some kind of closed toe/flatform type trainer.
wore Birkenstocks one year at the airport and had someone drag a case over my foot, it caught my big toe nail & ripped it off (was horrendously painful)
so now I only ever wear closed toes at the airport.

It really was! 😊

Glass and cutlery! Luxury.

I wouldn't have bare thighs or upper arms unless I am sure I'm sitting with someone I know.

I don't want my skin pressed against a similarly bare manspreader or XL stranger next to me.

Of course there are! I got bumped to business flying to Pisa once! DH sat at the back and I was in the front row with real glass and cutlery!!

They really won’t. Life isn’t an instagram reel.

We're off at a tangent here but and yes, even a fairly benign evacuation with no flames involved is a very rare occurrence but there can certainly be an issue with footware ,..

Obviously preferably no high heels but OTOH it might be handy to make sure what you wear on your feet is going to stay on your feet if you do have to evac...

Christ, why would anyone wear linen unless they're happy to be walking round like a used crumpled tissue???