Starting at the link above, click on the #21915 in the commit message, which takes you to https://golang.org/issue/21915.

So for no reason, it is about forcing code not to rely on the behavior.

There's a reason given.

As in, the reason has nothing to do with entropy, and it is just about breaking code relying on an unspecified behavior.

One really easy way to break any encryption scheme is to crack the random number generator. This works because random number generators are deterministic and you can get them to output the exact same stream of random numbers as they did before if you have the same seed / state. This is presumably about making the output of the random number generator harder to predict.

> This is presumably about making the output of the random number generator harder to predict.

Randomly deciding to add an extra byte of entropy only adds 1 bit of entropy. Adding the extra byte always adds 8 bits of entropy.

There is no cryptographic reason to do this. It is just to break assumptions about the API.