No official side chains yet. This is still being developed. Some challenges are technical and others are economical or governance related.
On the technical side, IBC (inter blockchain communication) is being developed in such a way that you will be able to transfer tokens from one chain to another with no need for a third party to coordinate and of course avoid double spend problem. This should be as efficient as possible and will always be less efficient than transferring tokens or processing transactions on a single chain.
The economical and governance related issues are, should we require each and every block producer to maintain and run the exact same number and type of side chains as every other block producer? If yes, it will be much simpler to operate the network but it is still not clear this is the right way to move forward. This will also put a burden on the stand by block producers and witness nodes.
Perhaps a more flexible side chains infrastructure is better where not all side chains need to be so tightly connected to the main chain.
So to summarize, there are the development issues of IBC which will enable technical communication between compatible chains and structural decisions which have many different economical and governance unsolved issues.