What Are Some Developer SDKs to Control Smart Outlets?

TP Link doesn't offer an official SDK to control their smart outlets and Belkin WeMo has sunsetted theirs I think.

The only popular options are IFTTT or Apple Homekit?

Not sure if IFTTT is the way to go. I'm planning on using it for a device I'm producing but won't be able to afford $199 a month.