The Rest API for issue relations lacks means for identifying changed data. The @modified@ timestamp on issues itself is not updated on a relation (which is okay since any old issue referenced by a duplicate would be pushed up the sorted list, otherwise), and issue relations can only be retrieved for (or worse, with) a specific issue.

So, if one would like to mirror information into (e.g.) Excel to do some Obscure Wizardry, one would - always - have to poll relations for all issues, one request per issue.

The ability to pull issue relations as list (with filter mechanisms alike issue lists) would certainly ease the pain.

However, I don't know how to allow delta requests and therefore to close the gap completely:
Since relations will be added and removed regularily, a simple @modified@ timestamp for relations (and the ability to pull "modified since" lists) would not do the trick; deleted relations would have to be stored as well and this type would need a special handling and possibly introduce too much complexity.