Product recommendation (mini display port hub)

That 2014 MacBook Air has

• A Thunderbolt 1 port with a Mini DisplayPort connector that can double as a Mini DispayPort
• Two USB-A (USB 3.0) ports
• A MagSafe 2 charging port

It might be theoretically possible to attach a powered Thunderbolt 3 dock to that MacBook Air with the aid of the bi-directional Apple Thunderbolt 3-to-2 adapter. Other World Computing sells such a product.

Other World Computing – OWC Thunderbolt 2 Dock Solution

Note "powered" and "Thunderbolt 3". The Apple adapter does not transmit power from one side to another and so cannot support bus-powered Thunderbolt devices. The Thunderbolt 4 and 5 standards do not support connecting Thunderbolt 4 and 5 accessories (like docks) to Thunderbolt 1 and 2 hosts.

That said, I cannot recommend that you spend $220 on such a dock.

• Your MacBook Air's Thunderbolt port is a Thunderbolt 1 port with a maximum speed of 10 Gbps, which will serve to bottleneck the performance you could expect to get out of a Thunderbolt dock.
• Your MacBook Air supports a single external monitor with a maximum resolution of 2560x1440 pixels. Adding a dock will not change that, even if the dock is capable of driving dual 4K monitors when attached to a newer Mac.
• You already have USB-A (USB 3.0) ports that are usable for attaching external drives.
• Many modern monitors have DisplayPort inputs – and those that are compatible with your Mac could be hooked directly to it using a simple, cheap, Mini DisplayPort to DisplayPort cable.

MacBook Air (13-inch, Early 2014) - Technical Specifications - Apple Support

Re: “Alternatively, a MDP(M) – USB-C(F) adapter would also be useful; interestingly, I’m finding several that go the other way, but, y’know, that’s the wrong way.”

I don’t think there is much call for adapters that go in the direction you want.

Most modern monitors that have USB-C inputs also have regular DisplayPort inputs. So you would just connect an old computer’s mDP or DP output directly to the monitor’s DP input with a passive cable.

Wirh the adapter you want, the interface to the computer would just be DP (a video interface), so you would not be able to make USB connections on the USB-C end, or have the monitor send power to the computer via USB-C Power Delivery.