One issue with doing a flat find and replace with this key (but also helps show why it is useful) is that people are tagging trails that are simple_walking as T2/T3 because they are exposed. Realistically someone looking at T3 that understands what that means would assume it’d be as rough as use_of_hands, which is what it would be changed to in a one to one comparison.
I feel like this key would exist on top of older ratings for a while and be used in an if hiking_technique, else sac_scale kind of way.
update: while not quite as OSMy as this proposal, I basically already had this written already for something else and just had to adapt it. RFC: hiking_exposure key (along with hiking_technique this disambiguates sac_scale)
So now there can be a simple_walking path with R exposure, a use_of_hands path with PG exposure, etc. SAC T2-6 exposure is pretty confusing on the OSM wiki, I feel like these basic buckets make sense
G- not exposed and no real risk of falling
PG - greater risk of falling, but still unexposed
R - indirectly exposed, falls could be serious but somewhat unlikely
X - fatal exposure, you lose traction you dead