All GSM phones have at least one of these (multi-SIM devices have multiple), they uniquely identify devices with SIMs and are held in databases shared intentionally amongst many nations for blacklisting and such.
If a phone is reported stolen in the UK and reported, it's IMEI can be added to this list and the device becomes useless in participating countries, say for example, Spain, or Germany or the US.
My point is, it's a globally unique identifier; tempering with, modifying or cloning them is illegal in some countries.
The SIM itself is almost irrelevant, but, with the information mobile providers hold, it's trivial to link a SIM account, a device identifier and a person (particular given some countries require ID by law to obtain a SIM).
Furthermore, being criminalized in some countries has caused discussion of how to change IMEIs to be censored in technical forums everywhere. The obvious draw is stolen phones, so nobody wants to touch the topic with a ten foot pole, despite its straightforward relevance to privacy.
That attitude reflects the dead ends I've experienced when looking around for how to change IMEIs for various phone models I was interested in. Also note all the disclaimers in the thread you linked.
Maybe recent phones are still so straightforward with QPST that any time the question is actually asked it's bound to get flooded with crap? It certainly doesn't feel that way. Eventually I'll get around to setting up another Windows VM and seeing what modern QPST can actually do.
If a phone is reported stolen in the UK and reported, it's IMEI can be added to this list and the device becomes useless in participating countries, say for example, Spain, or Germany or the US.
My point is, it's a globally unique identifier; tempering with, modifying or cloning them is illegal in some countries.
The SIM itself is almost irrelevant, but, with the information mobile providers hold, it's trivial to link a SIM account, a device identifier and a person (particular given some countries require ID by law to obtain a SIM).