@iBug as you've deleted your question, short comment here: whether you can avoid the implied factory-reset on unlocking the boot loader might be device dependent. There are rare cases where you can. One is here on my desk, where some XDA dev wrote an app to achieve that. As for the why: security, so no "unauthorized thief" can circumvent the lock access your data.

@Izzy I think it's time to undelete the question and you should add that as an answer.

@iBug It's still deleted.

But the main part is already answered in the linked question.

(I only know of a single exception so far)

@Izzy You could ask and self-answer a question specifically about that model.

@DanHulme Sure, I could. Would that be useful? It's an LG P880 from back in 2013. And what was done was some XDA member writing an app to do the unlock while booted into the main system (so it doesn't violate that safety guard as one needed to know the pin/pattern/password in advance).

The P880 uses "fuses" for bootloader locking – so once burned, no way back. That specific app calculated which fuses to burn in what order, so it enabled the owner to unlock/relock the device up to 8 times (instead of only once).

@Izzy About your captive portal detection question, are you just asking for a list of all the options and what they do, or is there a specific problem you're trying to solve?

@DanHulme Both. Along the lines of "tin foils": keeping the cp check in place but avoiding the Google server.

The full question behind the issue would probably be off-topic (how to setup and use your own captive-portal-check service), so I've kept to asking about the options only here – which should be on-topic as not restricted to devs but also relevant to power users.

And maybe not-so-power-users as well ;)

Being on it, I've just established our captive-portal tag-wiki (tag already existed, but abstract and wiki were missing).

Didn't include "hot questions" this time – the tag is currently used by 3 questions only ;)

Umm, sorry – number just increased #D

… to 17 now.