SIM cards cannot be disabled in dual SIM mode
Issue description, as found on Taiga
In Android 6 the "SIM cards" settings allowed to disable one or both cards to disconnect their networks without removing the card from the device. This feature is not available right now.
In Android 7, we released a first test version of the dual SIM toggles in a beta software release. However, we were not able to stabilize it enough for a general public release. Android 9 introduces major changes to telephony components compared to Android 7, so that we need to partly start from scratch here again.
LineageOS provides dual SIM toggles on the FP2 in Android 9, however, they don't seem to actually disconnect SIMs from the network when disabled. Therefore, we decided to not take over this implementation as of now.
Link to Taiga issue
Internal Issue ID