What is an impure NIST true random number generator? I'm working on a true random number generator, and I came across this phrase in the American NIST guide, in section 6.4.1 Non-Approved Conditioning Components:


  The entropy rate provided shall be no greater than the entropy rate provided by the input to the conditioning component; full entropy shall not be provided by non-approved conditioning components.


This suggests that you can have a random number generator that does not provide full entropy.  So if you use the de-rigour noisy diode source natively generating approximately 50% pure entropy, your generator can only output 50% entropy.  I'm unaware of any source that can natively generate 8 bits /byte entropy.  It would actually be undesirable  to run the source sampling equipment cranked up to 100% for many engineering reasons.

What is the point of a true random number generator that only produces half pure entropy?  Do you only use it half of the time?  Or do you just use every other byte (the random ones), ignoring the predictable ones?  Seriously though, is it just a thinly veiled warning that "it's our way or no way"?  Or..?
The NIST source you link to is SP 800-90B. If you look at the other two publications in the series: 90A and 90C, you can find explanations of how you can use a TRNG to instantiate a DRBG or requirements on how to condition the entropy.

Here is what section 7.1 of 90A says about entropy input to a DRBG:


  Ideally, the entropy input will have full entropy; however, the DRBG mechanisms have been specified to allow for some bias in the entropy input by allowing the length of the entropy input to be longer than the required amount of entropy (expressed in bits). The entropy input can be defined to be a variable length (within limits), as well as fixed length. 


So you can use a TRNG with 50% entropy rate to initialize a DRBG, you just need a longer input.



90C defines the assumptions more exactly in section 4.2:


  
  An entropy source output string containing $2n$ bits of entropy can be conditioned into a string of $n$ bits that contains full entropy output1 using an approved conditioning function, where $n$ is the length of the output block of the approved derivation function (see SP 800-90B). Note that in SP 800-90C, a derivation function2 is used as a conditioning function.
  A DRBG that has been instantiated at a given security_strength can be used to produce security_strength/2 bits of full entropy output if constructed as specified herein.
  


The approved conditioning functions are listed in 90B, after the section you quote. Anything else would fall under non-approved, but something sufficiently standard would be fine if you do not mind diverging from what NIST says.



So a half-entropy TRNG is very useful. You just need to take the entropy rate into account when using it.