Originally Posted by javabytes

This isn’t really correct. I’ve had iMessage problems/annoyances when disabling the SIM. It goes without saying you wouldn’t receive any SMS messages sent to you by non-Apple users… but when disabling the SIM, it disables the line altogether on your phone and disassociates it from your iMessage numbers on your Apple account. You cannot send or receive iMessages over Wi-Fi or other data connection using your phone number if the SIM is disabled; even if the local SIM is providing a data connection, you are not able to use that to “retrieve” the messages for the number associated with the other SIM that you have disabled.

Physical SIMs were treated differently if you physically removed them vs. disabling; Apple (historically, though there have been reports of difficulty lately) gave a 30 day grace period during which time you could continue to keep using iMessage and FaceTime despite that number not being currently associated with the phone. Not so if you turn off the line in the iPhone settings (whether physical or eSIM).

Disabling data roaming but leaving the SIM otherwise enabled is another option, though it of course won’t stop everything (such as phone calls and SMS that don’t rely on data).