Oh I'm sure there will be. It will be technically difficult (but not impossible) for them to allow other app-stores and sideloading but have the hardware and software be different enough in both markets to not have some slip through.
I suspect there will be lots of hacky shit for this.
I remember that there was an identifier based on model number. FaceTime wasn't allowed in the Middle East for a while, there was a way to tell if the model will support it based on the last character after the / in the model number. Middle East models won't even have the app at all.
Propably they'll do the same for models sold in the EU.
There are already hardware variants of the same iPhone. I think the US gets an iPhone with all eSIM, and China has two physical SIM slots.
I'm sure it will be and I'm also sure Apple will do EVERYTHING to make it almost imposible to do that. They may also block users from using their iPhones if they discover that they are set to EU, but they are actually not in EU.