Currently it seems easy enough to write clients/servers using the protocol and the Synapse server can easily be forked (https://github.com/element-hq/synapse) if you find the need to do so. It does seem to be mainly worked on by Element, so there's always a risk that they might stop working on it and instead work on a closed source implementation. That does seem unlikely though as it goes against their goals and the idea of an open, secure communication standard.