I am pretty sure their engine can handle implementing a proper reaction system. I am sure it can even be made with mods.
They probably can, the real question is exactly how. There's no prior precedence for a proper reaction system in DOS2. The closest thing DOS2 had to ready actions was the skill Reactive Shot, and it just allowed you to fire up to 3 arrows at enemies entering or moving within a specified area without a movement skill (which are also the prior precedence for the jump/disengage bonus action that exists in BG3).
If it's ultimately left up to mods like I suspect it will, it's not going to be as easy as you'd think. It's the kind of undertaking that would probably take a year or longer to figure out an elegant solution for, especially since it appears that this engine lacks any form of proper pausing. That's going to be the biggest hurdle to overcome, forcing the game to recognize when you can use a reaction other than a non-controllable opportunity attack, recognize if you even have the resources to use it (for reactions that require spell slots like Shield), and then interrupt an enemy turn so you can take that reaction.
