If corporate no longer wants to support mobile web, then it means I don't have budget to host it. It means developers can't put time against fixing it, QA aren't tasked to test it before releases, and support staff are not being trained in supporting it. The last one is pretty key because it's a huge metric for cost center: how many support calls is a thing generating? And if the thing is not supposed to exist anymore, then I would have to answer questions like "why is it still accessible?"
Internal job tracking metrics would have to answer why any time is going to running this thing, and god help us if there's a security breach via this endpoint we were supposed to have eliminated N time ago.
An unsupported internal API is one thing - and they're generally a huge timesink anyway. An unsupported external user interface is a cost center which I can't justify, and impacts numerous other parts of the business.