I just updated my iPhone to the iOS 26 public beta and was pleasantly surprised to see that you can now leave iMessage active on a switched-off SIM.

You still can't get calls or SMS, obviously, but now you can use that travel eSIM without having to choose between potentially getting roaming charges (plus more battery drain) or turning your home SIM off and getting kicked out of iMessage.

A really nice improvement for all of us Americans who have friends and family with iPhones that refuse to use messaging apps like the rest of the world.

My carrier doesn't support RCS on iPhones, so I don't know whether you can also keep RCS on a switched-off SIM.

Edit: You can still only have two numbers attached to each phone at a time--eg if you have SIMs A and B active, when you switch off B it'll let you keep the number in iMessage but if you switch on C and turn on iMessage it'll kick B out. Unlikely to be an issue for the usual use case of switching off your home SIM while traveling, but if you're juggling SIMs on a trip be sure to check in settings.