How to limit permitted combinations in a junction table

[Remster]

Agreed. Also, there's a current scenario where the actual supplier has offloaded all their support responsibilities to a third party, and it would probably be a good idea to record the details for both of them.

[orange]

Doesn't that allow for only one contact per system? We don't need to know who sweeps the floor or who updates security passes, but we do need to know who handles the contract, who manages the helpdesk, who's an expert in such-and-such aspect of the system, and so on.

Yes that is true. In table SystemQ you could have compound PK of (SystemId,SupplerId and ContactID) to allow multi suppliercontacts per system.

Just as there could be a change in Supplier, so to could there be changes in SupplierContact, or even supplierContact expertise. Although your primary concern is the current contact, current supplier etc, it seems you have need for some history also. This will impact your design.
But helpdesk, contract manager, contacts with various expertise are new requirements. Suggest you get the requirements under control and model and test/vet your evolving model.

[Remster]

But helpdesk, contract manager, contacts with various expertise are new requirements.

Hmm, not really. In my clarificatory post, I wrote "each system can have multiple contacts". This was precisely to cover contacts with different responsibilities and expertise (although it could also be used for retaining contact history).