This task is a tech spike to determine what technical requirements there are for us to add a new string type for notifications, to add to the existing HTML and plaintext variants. This includes capturing information such as:

1. What's required (and where) to define these strings to align with the existing implementation of the other notification string variants (such as any declaration of the string type to make the notifications system aware of its existence, if necessary).
2. What would be required within the SMS subsystem and/or SMS notifications functionality to support using this new string type.
3. Are there any risks, potential regressions we need to be aware of?
4. What would be required to have a fallback in place - for example if a notification was declared to support SMS but did not have SMS specific strings. This is not confirmed functionality (we may simply reject that case), but it would be worth investigating if it's trivial to simply fall back to the plaintext string in this use case.