Where in the source code of the Core Client is the GetBlockValue() function? I was reading Antonopoulos' book and there is a piece of code in chapter 8 in the part about Coinbase Reward and Fees and the code is a GetBlockValue() function. I was looking for it on github but didn't find it. I found something about a function called GetBlockSubsidy() that works in a similar way and is apparently a newer version but I can't find this one either. Also all the links I found linking to main.cpp in the repository don't work. Where is this part of the code now or what am I missing?
GetBlockValue was renamed to GetBlockSubsidy, and main.cpp was renamed to validation.cpp.  So you should now look at GetBlockSubsidy in src/validation.cpp.

You can search on Github for GetBlockValue; you won't find any hits in code but you will find some in commits.  That's how I found this.