Brotli support in API clients are quite low. I run a small API service, and you'd be lucky to see API clients even using gzip.

True, though given its relatively easy to support you could get some savings in the few cases that do use it.

Also if they own some/all of the SDK's that are used for hitting their API they could bake in brotli compression at that level.

I don't think there would be many clients that would support that. The API clients are usually not browsers in their case.