What are Gadget and Gadget denition namespaces used for? We, on trwiki, have not filed a request to add those custom namespaces, so is there a way to remove them?
This was an intentional change in https://phabricator.wikimedia.org/rEGADe6b8b57d9b0df628aa912c7e4f54990652d221be so they are not "custom". As far as I know there are no plans to allow removing them (@Legoktm might be able to elaborate).
If there are convincing arguments how these default namespaces negatively impact your workflows and why that negative impact is big enough that it justifies resources to long-term maintain additional code which allows disabling, I'm pretty sure that developers would be open to discuss such arguments.