Well, I am not in the position to decide what needs to be done.
The possible solutions are:
1) A soft-switch (as currently) which actually incorporates the entire screen and doesn't care about aspect-ratio
2) A soft-switch (as currently) which actually incorporates the entire screen and does letter-boxing
3) A hard-switch
4) A combination of all the other three, up to the developer to chose from
Personally I prefer (1) or (3) as (2) is something the user needs to implement himself also on all other platforms if not using a native aspect-ratio and therefor it's not a business of SFML. Version (3) gives a damn on Apples guideline (or doesn't, depending on how you want to read/interpret them). Version (1) would work quite closely to the current implementation, but it would actually use the entire screen. Version (2) would give the user/programmer an unexpected result since that is not what happens on Linux or Windows iirc. The resulting solution should of course be consistent to the other OS-specific behaviours.
I could possibly implement it myself and make a PR, but that would be useless until there's a decision made by the SFML-team on which solution is the desired one. (4) is not something I might be willed to invest my time in as it would be too much time-consuming.