can confirm with Octave 9.2.0. but i don't think the semicolon has anything to do with it (or did you mean the colon).  Matlab is able to use that syntax for object/field creation. Octave can only use that syntax for an existing field/object:

>> A.a{:}=1
error: invalid dimension inquiry of a non-existent value

>> c = {99}
c =
{
  [1,1] = 99
}

>> A.a=c
A =

  scalar structure containing the fields:

    a =
    {
      [1,1] = 99
    }


>> A.a{:}=1
A =

  scalar structure containing the fields:

    a =
    {
      [1,1] = 1
    }


>> B.c = 1
B =

  scalar structure containing the fields:

    c = 1

>> B.a{:}=1
error: invalid dimension inquiry of a non-existent value

>> B.a(:)=2
B =

  scalar structure containing the fields:

    c = 1
    a = [](0x0)