Seconding that by sticking to a turnbase system, BG3 has already prioritized mechanics over cinematics for combat. Turnbase combat is inherently non-immersive - characters taking individual turns is something that fundamentally breaks reality. As it stands, there is nothing done with BG3's combat animation or camera angles that make it any more cinematic than Solasta's. Both companies have chosen to keep combat visuals mostly functional, which is a design choice I agree with (as impressive visuals can quickly wear thin when constantly repeated).
How many times have you seen such a dialogue box in BG? Or any other cinematic rpg for that matter. It's rare, because it breaks immersion. It's very common in strategy or tactical games. RPG's are controlled almost entirely by context dependent menus and hotbars.
Here's the thing though, we've already seen this implemented in game. Dialogue in BG3 is far more cinematic than combat - yet Larian has implemented a "visual interruption" with the skill check dice rolls.
Every time we meet Astarion and he puts a knife to your throat (which is far more cinematic and specifically animated for that moment), the game waits for you to make a roll while the two of you rock back and forth in an endless struggle.
On top of this, even if we accept that Larian does in fact want to prioritize cinematics, other AAA games that are far more cinematic than BG3
have implemented various forms of "visual interruptions" for years. This is essentially what Quick Time Events are.
I just wonder how people asking for reactions would expect them to be depicted graphically? How do you know when to cast shield if you can't see the dice roll? How do you counterspell when the act of casting a spell and its effect are depicted as almost instantaneous like in BG3? And featherfall, how would that even work as a reaction graphically? In Solasta, if you don't tick the option to have a timer for reactions, the game pauses until you click yes or no. Can you really see Gale, having been pushed off a cliff, frozen in time as he decides whether to cast the spell? That happened to me once with featherfall in Solasta. Time just stopped as I answered the phone.
Making them similar to the dice roll pop-up that interrupts the dialogue cutscene (but a bit faster to fit the combat), would be a good starting point. The alternative is something similar to the tutorial pop-ups they've just started implementing, if we want a less intrusive implementation.
In terms of triggers for specific spells/questions:
1) Shield - since information is freely available in BG3 (no metagaming issues), the reaction to shield should only prompt when the +5AC can prevent the hit
2) Counterspell - slow down or freeze time as the enemy is doing their incantations, show prompt
3) Feather Fall - slow down or freeze time as character is falling, show prompt
And lastly, there's nothing stopping Larian from keeping the current options too. Instead of simply having "on/off" for reactions, the reaction prompt can be the 3rd option - i.e. "on / off / prompt me".
This way for characters with less reaction options (aka the martials), you can just keep opportunity attack turned on instead of manually clicking each time.
My bigger concern of why we might not get proper reactions is similar to Saito's, in that the current engine doesn't support it. For example, it doesn't seem like the current engine support a "proper pausing" at all. There is currently no way to pause the game at all without quitting. Menus don't do it. Entering turn-base mode outside of combat is still a clunky mix of real-time and turnbase.