I don't really see an issue for manufacturers there. We have the same regulatory issue with Wi-Fi power which will adjust transmission power differently based on the country it's sold in. The phone could easily enable to disable a feature depending on where it is. Remember Bolivia dBm limits ? :)

Regarding your German link. It's about tampering with evidence which I suppose means it's about changing the IMEI after a crime to hide yourself. I suppose that would be illegal everywhere?

Regarding your UK example, the law in question (https://www.legislation.gov.uk/ukpga/2002/31/section/1) mentions circumstances and gives exceptions:

" But a person does not commit an offence under this section if—

(a)he is the manufacturer of the device, or

(b)he does the act mentioned in subsection (1) with the written consent of the manufacturer of the device. "

So I guess it's okay in the UK if the manufacturer allows it and is okay with it?

But of course I'm not trying to promote this feature for illegal use but mostly to hinder "inescapable" tracking from Advertisers , Mobile Operators, ISPs, Phone Manufacturers and ...

True, in some regions this is not allowed. It was mainly for the purpose of blocking stolen IMEIs though which is not really a thing anymore (since Apple/Google's activation lock is so good there's no more need).

Also, for avoiding tracking purposes there is really no point in changing your IMEI. There's many other ways they can track you. In fact I don't see any real reason to do it at all.

How would a cop even know about this and arrest you? Being serious.

Easy: check if the imei on the back matches the one shown in the software as part of a theft check. Threaten the user with seizing it for suspected theft or whatever if they refuse.

That would only work if you are already under suspicion for something. I don't think many random cops knew how to check the IMEI, also because you'd have to open the phone up. (I think so?)

The german link has no source.