Wearable transceiver safety considerations
If you read the page you linked carefully, it doesn't say that you can't use these things within 20cm of the body, but that Specific Absorption Rate testing is required for regulatory compliance if you do.

If the datasheets are telling you not to use the devices within 20cm of the body, most likely it's because they don't want to be responsible for the regulatory issues involved, especially since they have no control over factors like what kind of antenna you attach.

For personal / development / testing use, I can hardly imagine you would be likely to endanger yourself by using Zigbee or Bluetooth transceivers within 20cm of your body. This simple rule is likely to keep you safe: If your body starts to feel hot near the device / antenna, stop transmitting or move the device away from your body.

But if you're planning to sell these devices, there will be no avoiding a SAR evaluation for regulatory compliance. That's above my paygrade, but I doubt you'll have that much trouble with it, as so many of these devices exist; just make sure you actually do your homework on what is required of you.