I would untag the previous post as “solution”, and rather redirect any interested person to ✏ FP3: unlock the bootloader or to https://private.z3ntu.xyz/FP3/verify-code.php.
These are the facts:
- When FP3 was released, there was no need to enter any code to allow the bootloader to be unlocked
- In a successive update, this new screen was introduced by FP, and some reverse engineering was required to unlock the bootloader (the above cited guide was then posted by the community)
- Now, FP makes available its own tool to circumvent an annoying dialog box that they introduced themselves, instead of simply removing that dialog box in a successive update
I have not checked whether the IMEI and serial numbers are actually sent to FP servers when you use their tool or whether the decoding is done client side, it would be interesting to see. (Edit: it has been checked in a later post by justlurking that this data is indeed processed by a remote server) But I let you draw your conclusions.
If I brick my phone, I will consider myself the only responsible for it, and will not open a warranty case with FP. But if there is a hardware failure, I wish FP to respond for it, independently of whether I have at some point intended or not to unlock my bootloader.
Moreover, it is not FP’s business to know whether or not I have the intention to unlock my bootloader. Well, I really don’t know (yet) what the FP tool for getting the code for unlocking the bootloader actually does, but in order to keep on the safe side, I prefer the strategy suggested on the above cited links.
Edit: it seems that there are actually quite a few guarantee-unrelated good reasons for which FP might want to collect, or even might have to collect this data. But using an alternative method to get the unlocking code still does not hurt 
.