I think this is only true for older mobile networks. In 4G and 5G, I don't think the IMEI is part of any unencrypted radio message anymore.
Even the IMSI is only used when absolutely necessary, i.e. for the initial attachment procedure when cold starting a device or entering a new routing area; after that, it's replaced by an alias called TMSI to make tracking phone users a bit harder.
New Android versions will supposedly have a switch in their settings to show a warning every time the IMEI or IMSI is transmitted in plantext [1].
Even the IMSI is only used when absolutely necessary, i.e. for the initial attachment procedure when cold starting a device or entering a new routing area; after that, it's replaced by an alias called TMSI to make tracking phone users a bit harder.
New Android versions will supposedly have a switch in their settings to show a warning every time the IMEI or IMSI is transmitted in plantext [1].
[1] https://cs.android.com/android/platform/superproject/main/+/...