I don't know if they could do a try/fallback or if one is available as a setting (I haven't looked), but apparently they don't by default; Scryer Prolog:
?- X = '.'(1,'.'(2,'.'(3,[]))).
X = [1,2,3].
SWI Prolog 9:
?- X = '.'(1,'.'(2,'.'(3,[]))).
ERROR: Type error: `dict' expected, found `3' (an integer)
> Code written 30 years ago which uses the dot in the old standard way might trigger SWI to try and read it in the Dict.key way and fail.
Why would they do that? SWI 7 reading SWI 6 or ISO prolog code should be rather trivial, shouldn't it?