I would have preferred 14 Dex and either 10 Int or 12 Str for Shadowheart, but I can see why they refrain from ideal builds. Definitely an improvement both for Shadowheart and Astarion though.
Stats in D&D mean to resemble a character as close as possible, and it's good to see that they're willing to experiment and shuffle a few numbers around. I hope that they don't sacrifice too much gameplay balance to achieve that. Personally, wouldn't have listened to the forum crowd here that demanded more dexterity on shadowheart, because clerics never relied on DEX for their armor class, they get that through their actual armor, that usually limits the AC you gain from dex to 1 or 0.
Later on, all that dex will be well for naught, maybe you're lucky and you find a magical breastplate that has no dex restriction on it, but until then, you're missing out on stats. And even worse, they (Larian studios) traded away her high strength for that, which IS something that clerics could use, they may not be epic kill-death warriors, but they can dish out a good whack every once in a while. Not shadowheart though, will be very hard to hit anything with no strength bonuses, and whatever hits come through, will be far less potent than before.
At least wisdom remains in a good place. It's the most important stat for a cleric, and as long as she hovers around 16 wisdom and some decent consitution, she'll remain serviceable, though I do hope they get her strength up, 12 or 14 will be sufficent to fulfill the occassional melee support.
The only way this makes sense to me is if you think BG3 is using 3e DnD.
Trickery Clerics are limited to medium armor and consequently this is a good change both for early gameplay and throughout the adventuring career. She would have had to use a feat to use heavy armors w/o penalty and this would *NOT* fit well with her being a stealthy Trickery Cleric who contrary to your claim are best served with higher Dex - esp. if they are supposed to be "melee support"/tank. +3 AC is HUGE in 5e. Strength is usually regarded more of a dump stat for this domain. Shadowheart can use a finesse weapon which will allow her to use her high Dex for hit/dmg, using a dagger instead of a mace will average -1 dmg per round - hardly "far less potent"! The only concern is losing +2 attack roll on touch spells like Inflict Wounds (this is alleviated by the ease of getting advantage however). As far as I know, 5e DnD there are no special armor properties that removes Dex restrictions (ie. Mithral armor simply removes Stealth disadvantage from armors in 5e).
This change was highly requested for several reasons; narrative dissonance, physical dissonance, and low combat efficiency/survivability. Statistics show Shadowheart had the dubious honor of being the only companion more likely to die than the main player character. This change doesn't "sacrifice" balance, it establishes it as Shadowheart was notoriously weak unless exploiting the lax rest mechanics.
