Disc Brake Mounts

I'm looking at a new, high-end steel frame that has accepts a flat mount but has an IS mount at the fork. Should this concern me? Is the IS mount future-proof?

You can make them work with drop bar hydraulic levers but its not as straight forward as flat mount.

I’m very surprised that anyone makes an IS mount in 2021, I assume you mean post mount.  Either way getting brakes for it will be a pain, so if it were me and I really liked the frame, I would get it and just find another fork.

IS mounts are definitely not future proof since they pretty much don't exist any more...lol... But, if you meant post mount, then you can get the Hope RX4 in a post mount to work with either Sram or Shimano drop bar levers. And, FWIW, they are awesome additions to a gravel bike. 4 piston power with excellent modulation.

Here is the frame in question:

https://shop.circles-jp.com/en/products/crust-bikes-nor-easter-frame-set

I know it takes a mountain crank ... don't know how that effects things (sorry for my ignorance - I tried researching this question and ended up even more confused).

So IS mount is dead? 

There might be IS to flat mount adapters now

That is a strange combination, flat mount rear and IS front.  You can get an IS to Post Mount adaptor.  However, for this sort of frame, I'd think flat mount is the future.  Could you build a great bike with that frame? Absolutely.  That said, I think you will find that similar frames, such as a Surley Disk Trucker, would provide the same capability, but have more up to date fork fittings.

I agree with @GravelInSipsey. To me, the front half of this bike is from a different decade than the back half. 1 inch headtube and IS brake mounts is incredibly strange in 2021. It would have been a little strange 10 years ago. This seems like a poorly copied Surly Disc Trucker.

IS mount is not risky at all to buy. First off, you'll be using an IS to post mount adapter. The big advantage there over flat mount is you can run rotors larger than160 if you want just by using the appropriate adapter. This comes into play more so with mechanical discs. There are a bunch of good mechanical disc brakes available for post mount- TRP Spyres, Yokozuna/Juin Tech, etc. If you want, Crust sells a premixed pair of the Yokozunas 2-piston brakes- one flat mount, one IS/post mount.
Now if you want road hydraulic, Shimano GRX/road levers can be used with post mount calipers. It's been a while since I looked into it, I believe Ultegra and XT calipers both match up with the road levers. Another option (which I have for my Crust Bombora) is a A.S.S. IS to flat mount adapter, so you can simply mount a flat mount brake to an IS fork.  https://www.assolutions.ca/shop/adapters/flat-mount-adapter-for-is-frame/ I believe when I set it up it used a 203 rotor. 
Bottom line, for flat bar setups, IS mount using a ubiquitous IS to post mount adapter is current tech. For drop bars, IS is now legacy tech like cantis, but still easy to do a setup.