No, every version of .NET has been standalone. You can install .NET 3.0 with no other version installed, ditto .NET 3.5, ditto 4.0, ditto 4.5 etc.
Now there have been fewer versions of the CLR than there have of the .NET framework overall, but that's a different matter - and doesn't change whether or not you can install .NET without installing anything else.
Likewise some versions of .NET effectively install over the top of others (installing .NET 4.5 when you've got .NET 4.0 installed replaces the .NET 4.0 libraries) but you can still install each version without installing anything else first.