Yes the loophole still works but it is not quite as simple as just swapping SIMs around and calling Customer Care.
Always best to do a restore as new on the original iPhone with a new Mini-SIM and follow the iTunes steps to select the correct $20 data plan which includes the 200 SMS Txt message plan. Once you get converted to the $30 iPhone Unlimited Data plan you will also have the $5 200 SMS Txt message plan unless you want a different SMS Txt plan. Note this is also a way to get the $10 1000 SMS Txt message plan as that is still an option with the iTunes activation on the original iPhone.
Here is a thread the covers the journey to the iPhone Unlimited Data plan. You will have to work for it and use iTunes. If you call, your chances of it working are quite random as most first line reps will not be able to see the feature:
iPhone Data Plan + 200 Text - MMV1 ( SOC - original iPhone data plan ) even when you provide the IMEI of the original iPhone.
https://forums.macrumors.com/threads/1124040/
Dave