For a Mendix 11 app, I would not recommend disabling the React client just to support one legacy widget.
I'm sure that, in Mendix 11, the React client is the default for new apps, and the legacy Dojo client is deprecated. Mendix also states that Dojo-based widgets do not work in the React client. In migration mode they are only replaced by placeholders so the app can still build, which is useful for transition, but not a real production solution.
So for your specific case, the most future-proof approach is: build or adopt a React/pluggable signature widget. :)
Mendix positions pluggable widgets as the successor to custom widgets, and for web they are React-based. That makes a custom pluggable widget the cleanest long-term solution for a signature use case in a React-client application.
About the current Marketplace Signature widget: the official widget documentation describes how it works, but I could not find evidence in the current Mendix docs that it is React-client compatible. More broadly, Mendix explicitly says that not all Marketplace components are React-ready yet and refers developers to the React status documentation to verify support.
So my recommendation would be:
Use the React client and keep your architecture aligned with Mendix 11.
Do not rely on migration mode as a production workaround.
Do not switch the whole app back to Dojo for this one feature unless this is only a very short-lived temporary fallback and you fully accept the migration debt.
Prefer a custom pluggable web widget wrapping a standard React signature library and storing the result in Mendix as an image or base64, depending on your model.
If this resolves your question, please mark it as accepted.