I’m not one of the authors of the proposal, but for background, I voluntarily worked in a blind school for 3 years in the past.
My assumption is that the reason, not to put tactile_paving=* on areas, is that the information is only really useful on ways, and the only interpretative information would be “there is some sort of tactile paving, somewhere here”, but now exactly showing where.
For platforms mapped as an area, I’d say the best option would be to have a highway=footway way in addition to the platform area, that can then be used for routing, both blind and non-blind people, much like it’s common for pedestrian roads mapped as areas.