Once migrating the it-orders case mgmt example from the legacy designer to the new one, an issue appeared: It seems there is a validation error on the process, the example is using two signals with symbol ":" and spaces.

This warning can be ignored and the process can still be saved, but you can't recreate it from scratch as the editor will prevent you from creating a new signal with colon or spaces in it. The fact that we use the node name as the signal name in case management (to trigger that node) makes this a more severe issue as you're more likely to use spaces and possibly other special characters in node names. The underlying XML does not have any restrictions on signal name (xsd:string), and the engine also accepts any string afaik, so as long as the parser can support it as an attribute value we can support more characters.
<bpmn2:signal id="_825e5b59-8a03-369d-b7ed-e5e3824400c9" name="Milestone 2: Order shipped"/>