This is MTR's proposal for the West Rail. Recall that MTR had to bid against KCR for what is now known as the West Rail.

I can't seem to find the correct name of this hypothetical spur line given the significant SEO "masking" by the actual Northern Link.

If you are interested in "never realized" railway mapping, maybe you are interested in looking at the Northern Link. The KCR proposal sat inside OSM as `proposed:*=` for many years until recently MTR went ahead with essentially a separate project from the West Rail. That Northern Link KCR proposal is "pinned into existence" by the West Rail relation, which contains all of the KCR's proposal on the "Greater West Rail" (unrealized).