Since we're starting to have quite a few pointers, I would suggest two options:

1. Start using type Null[Type] struct types where Valid is set to true/false
2. Separate the fields so that there is Deadline time.Time and DisableDeadline bool (as an example)

For 1. we could either maintain our own package of types used by the API, or rely on pre-existing work. One example is https://github.com/guregu/null/blob/21596e8e52a426a14cd73b22f6ffa4d31165302f/time.go#L12-L16. These would support JSON (de-)marshaling and be slightly safer to use than pointers (and dereferencing).

For 2. first I would like to ask: Why does the frontend need to pass null vs undefined? If we don't care about changing the value (disable), then undefined should be just fine. In TypeScript this would simply read deadline?: string which matches the API contract when using JSON omitempty (edit: Time might require a custom json marshaler to be omitted though, but I think that'd be worthwhile over using a pointer).