I have seen the wiki page and I did not mean “adding additional definition now” but in general. The additional definition “impossible” has been added to an existing tag some time ago which I believe was not a good idea due to the given reasons.
I looked around the vicinity of mine and found lots of foot=no that are not signposted or backed by some law. Of interest for this topic those, where the mapper thought, that walking there is a bad idea. E.g. along busy highways without shoulders nor verges. If it wont kill you, you will certainly raise bad temper with the motorists.
There seems no other way to map such and I am fine with the mapping as is. For the wiki article, it might get difficult to find a good wording though.
You (both @SomeoneElse@SK53) are right; I personally am only fine with this tagging “foot=no” where the mapper thinks, it should be forbidden to walk, while this is not the case, as long as this does not spread beyond the reasonable. Perhaps “foot=discouraged” suits better there?
This topic saw another case for “foot=no” under the “impossible” clause: @Matija_Nalis mentioned seasonal paths: In my mind, there the conditional actually applies to the path, not the access thereof?