Introduction Unearthed Arcana #7 features updated playtest versions of 5 classes: Barbarian, Fighter, Sorcerer, Warlock, and Wizard, matching the list from UA #5. UA #7 also features revised spells, another round of revisions to weapons, revisions to the Ability Score Improvement feat, and revisions...
I always enjoy the RPGBot guides, and this is no different. It's a good breakdown of the One D&D updates for Barbarian, Fighter, Sorcerer, Warlock, and Wizard.
A sidebar under the World Tree Barbarian specifies that the 2024 PHB will clarify that you can voluntarily fail a saving throw. The 2014 rules never so much as implied that you could do this, so it’s not so much “clarifying” as “introducing the idea in the first place.”
Huh. I personally allow most saving throws to be failed except for CON saves, as long as it's physically possible to do so. DEX and STR saves being the easiest to understand how to intentionally fail (don't resist, don't dodge).
I thought you just counted as a willing creature for INT, CHA, and WIS saves if you decided to fail the saving throw. I feel like the only Attribute it intuitively doesn't make sense for is CON like in the house rule you described.