It is indeed now widely accepted that Sparta was not a fossilised society from the so-called Lycurgan reforms. Instead, scholars now believe that the Spartans, or those in power, reacted to developments in Sparta's situation, such as going from a Peloponnesian regional power to an Aegean-wide regional power, by creating new laws and attributing them to their mythical lawgiver Lycurgus to legitimise them and make them more palatable to the Spartans. I go into the details a bit here. It is safe to say that much of what we know of Sparta was likely not factually reliable, but has come to us through a variety of filters, from the Spartans' own propaganda and invented traditions to moralising writings of Athenian Lakonisers like Plato and Xenophon.

The key secondary source discussing this phenomenon is Flower's 'The Invention of Tradition in Classical and Hellenistic Sparta', in A. Powell, and S. Hodkinson (eds.) Sparta: Beyond the Mirage (London, 2002), 191-219.

Flower is using a concept developed by Eric Hobsbawm and Terence Ranger (I recommend reading the introduction for an overview of the theory).

Edit: see also Anton Powell's first chapter in this book too.