Requiring common-disconnect (but not common-trip) breakers I think was a requirement since before 1998, but because it only needs common-disconnect, a listed handle-tie between the two breakers will suffice. (In other words, this should be a quite inexpensive situation to remedy. $8 for 3 of them at Home Depot for one particular common panel: https://www.homedepot.com/p/Square-D-QO-Circuit-Breaker-Hand...)
The handle-tie would get you common-trip too, wouldn't it?
If you have a breaker in your hand (now or at a home center), you can slap it briskly against your thigh and it will “trip” allowing you to see how the handle will look when tripped. To a cursory glance, it will appear on (but you will notice the handle can be moved about 1/8" more than normal).
That small movement is not enough to induce a trip or disconnect on the handle-tied breaker. (The requirement is only "Each multiwire branch circuit shall be provided with a means that will simultaneously disconnect all ungrounded conductors at the point where the branch circuit originates." That's to allow for safe servicing.)
* I own a house that was built in the early 20th century. As was typical of the time, the house has a small number of fused circuits, which do not have a ground wire, and where the wire gauge is thinner than would be required for a modern circuit of the same capacity. Just the same, it's perfectly legal to: * Add new outlets to existing circuits without changing the fuse to match the wire gauge of the circuit or adding a ground wire. It has to either be a NEMA 1-15R receptacle (no ground plug) or a GFCI receptacle with a label that says there's no equipment ground. * Replace the fuse boxes with modern breaker boxes without re-wiring the house to have grounded outlets. The only limitation is that the new breakers would have to be smaller than the existing fuses so that the wire gauge meets current code for the breaker size.
* I have a 1998 house which has several pairs of circus on opposite phases which share a neutral wire. This is legal still, but requires that the circuits are connected to dual-throw breakers, so that it's not possible to have one circuit hot with the other dead. But this house doesn't use dual-throw breakers like it should. As far as I know, that wasn't even code when the house was built. But electricians can still add new circuits without fixing that.
* Some years ago I was remodeling a 1956 house. Initially, the inspector asserted that we would need to replace a staircase whose width and pitch did not meet current code. But he was mistaken. Even though we replaced the stair treads, because we didn't make any changes to the structure of the staircase, the head county building inspector agreed that we didn't need to bring it to current code.