Yes. However the frequency it blocks is ~2.45GHz which is the same frequency as WiFi, Bluetooth, etc. and used to be the only other antenna other than the cellular antenna, where the frequency ranges from 600MHz-2.5GHz.

This used to be good practice because you would first remove the sim card disabling the LTE communication, unless the hardware was compromised, and then place it in the microwave to disable all other signals.

With the introduction and proliferation of eSIM on both devices and carrier sides, removing the SIM card no longer provides much protection and the additional of many other communication methods, most notably 5GHz 802.11x, the microwave trick doesn't really do anything either.

But it used to work.