Consider, for example, the following queries. These unexpectedly return different results. The first produces a warning and gives interwiki results, while the other two produce a warning and an error, giving no results at all. In all of these cases, both the warning and the error are inappropriate if we consider that the iwurl parameter was targeted towards ApiQuery.
https://en.wikipedia.org/w/api.php?action=query&iwurl=&titles=fr:Albert%20Einstein&prop=iwlinks&iwprop=url
https://en.wikipedia.org/w/api.php?action=query&iwurl=&titles=Albert%20Einstein&prop=iwlinks&iwprop=url
https://en.wikipedia.org/w/api.php?action=query&iwurl=&titles=fr:Albert%20Einstein|Albert%20Einstein&prop=iwlinks&iwprop=url
This is definitely an edge (corner?) case, but conceivably, a bot might be provided an arbitrary list of titles such as the above and then try to run this type of query on them. Given that iwlinks' iwurl parameter is already deprecated, perhaps it would be best to remove it sooner rather than later.