Formerly proposed subway line: OK to remove?

Someone put in a lot of work laying down the “Pink Line” (AKA “La ligne rose”) of the Montréal Métro and it’s tagged as Proposed, so it doesn’t show up in the normal views, but it makes things confusing when editing the map.

It will probably will never be built; it was just an idea put forward by one of the political parties during the campaign for the municipal elections in the autumn of 2017 and, since then, it has become pretty clear that it will never happen because the cost would have been prohibitive and other projects have already been launched instead. (BTW: The main reason why the cost would have been prohibitive is that it would have been a “diagonal” line. Whereas most tunnels of the existing subway network in this city are located under streets, this one would have had to be much deeper, in order to run far under buildings.)

Would it be okay to delete this feature?

If so, how would one go about gathering all the elements that make it up, in order to perform only one proper deletion?