Will steel frames rust if left outside all year?

Buy something cheaper and use the difference between it and the Ritchey to buy one of those lean-to bike sheds - the monster green steel ones spring to mind. 

Though the Ritchey would be a fabulous bike you are spoil for choice in the quality steel market if you shop carefully, especially if you include pre-loved.

my tuppence worth, and a bit of reverse logic from the norm

When it comes to winter bikes or those that need to be stored outside then why isn't the norm carbon - it is less affeted by the rain/salt/general crud and doesn't corrode (from the inside or he outside), so its only the groupset that needs to kept as clean

I have heard that bringing a steel bike inside a nice centrally heated home can cause internal rust because condenstion forms on the inside.

There are specialist aerosol Frame Protector products out there.  You squirt it in all the holes and slosh the waxy paintish stuff around.

J P Weigly was the one, but the brand was sold.

Ok I realise hardly anyone leaves their nice steel bike outside but honestly, the amount of snarky comments I've seen by steel bike owners over the years towards plastic carbon bike owners due to bike longevity etc etc etc...wow, this is an eye opener 😉

Not having a go, I love a nice steel bike, didn't realise you had to be this careful though (again, I suppose people rarely leave them outside that much tbf)

No way I'd leave a £2,500 bike outside for any amount of time, not just because of corrosion but because of security.

Ritchey do a breakaway version of the Outback which is designed with traveling in mind and comes with a travel bag. How about buying that version instead and then storing the bike inside your home in the travel bag?

It'd be a faff having to dis/reassemble the bike each time you want to ride it but I'd far rather do that then leave it outdoors.

Yes. Without question. It might take a while, but it is inevitable. It's not just the frame either. Your chain will go first.  Then the anodized coating on all your shiny allen bolts, then the rest of the drive chain...then your bike!

I have a steel Gios that has been treated internally against rust, but I still don't leave it outside. I'd sleep outside myself first....

Short answer: quite possibly/probably.

Longer answer: assuming not stainless steel. Intact paintwork protects the outside of the frame. The inside of the frame should have cataphoresis treatment, though it's generally recommended that's applied annually. If a bike is left out in the elements for a long time, there's a good chance that water will get into and potentially hang about on the inside of the frame. This can cause corrosion over time. More to the point, it's often corrosion you can't see until it is catastrophic.

I personally wouldn't store anything other than a proper beater bike outside. You are drastically reducing its useful life.

I have had a steel frame (Claud Butler) rust through in the past - found out by putting my finger through the seat tube!

It's replacement, also steel, has spent much of the last few years as my lunchtime shopping bike, stored in the outside bike shed at work. Drive chain has rusted, but, so far, that frame is ok. Will report back if we ever return to the office!

There's more vulnerable bits of steel on a bike than the frame. As long as the paint's good there shouldn't be many issues as long as you do your maintenance, some sort of cover from rain would be advisable though 

Well my old MTB has lived outside for much of its life and if rust was such an issue, it'd be a heap of orange coloured dust now. 

Clean your bike regularly, spray WD40/GT85 over exposed parts and don't worry about it. Yes, you may get some surface rust but not so much that it'll be an issue. As long as the steel is good quality, it won't rust to nothing.

A major reason in why Fiat (and Lancia) cars got such a bad rep for rust in the 70s was because Fiat traded the rights to the 124 model to the Russians who turned it into the Lada. In exchange Fiat got a load of very low quality steel that was ultra vulnerable to corrosion.