SK53
This is better addressed as a changeset discussion with the person who tagged them this way. The tagging is unconventional, but I think it's meaning is clear. 

It may also be worth checking SomeoneElse's lua script which will show if this is fairly common in the UK.

My guess is that a) either they prefer all footpaths to be shown the same with Carto-CSS or b) they had routing problems with highway=steps.