Patch 1.21 Notes

Confirmed by official sources — compiled from the developer's launch announcement for this update.

Hallowed Concepts DLC content

2 new Survivors: Enforcer (unlocked by default) and Acolyte (unlockable)
One new Curio: the N'Tormat
3 new enemies, 3 new bosses, 1 new elite type
3 new standard stages and several new trial stages
6 new interactables
2 new item types: 14 Hexes and 11 Hallowed Items
3 new standard items (one lunar equipment, two boss items)
New unlockable Survivor skins
New music by Alloyed Collective composer Stavros Markonis, featuring a track by Nando Garza

An optional Karma system that makes the final confrontation on the moon significantly more challenging

Halcyon Shrine: much less likely to spawn on Stage 1 (roughly half of runs get it); now consumes stage spawn credits on Stage 1 (0 → 40, Stage 2+ stays 50); siphons gold faster (5 → 7 ticks per second); spawns an extra golem (2 → 3); Alloyed Collective red items are rarer from the shrine and all red items are rarer from the Stage 1 version; drop tables downgraded on Stage 1; Halcyonite no longer drops gold
Solus Scorchers spawn less often, and basic Solus enemies (Scorchers, Extractors, Mines) have 50% less armour
Solus Mines no longer damage players when players kill them
Xi Construct AI now sticks closer to players instead of hanging back where melee Survivors cannot reach it
Gunner Turrets can now be repaired after they die
Engineer's Bubble Shield now blocks explosions from outside the shield, including Voidling's laser; the same fix applies to Operator's Firewall
Reduced the chance for Titanic Plains and Distant Roost to be picked as Stage 1
Reduced screen shake on the Junk Cube, the moon exploding, the Solus Validator and a few other things
The tar waterfall in Abandoned Aqueduct (and its Simulacrum version) no longer blocks pings and applies the tar debuff again when you jump through it
Targeting equipment (Recycler, Royal Capacitor, Gnarled Woodsprite and others) is more accurate in multiplayer
Prime Meridian lightning VFX updated so it is clearer that the damage is a cylinder rather than only near the ground

Commando's Double Tap attack speed cap raised to the intended value, and it now fires from both pistols for other players online
MUL-T's Retool no longer puts the first equipment you pick up into an unreachable slot
Mercenary's Eviscerate passes through allies instead of targeting them
Railgunner's Magnetic Accelerator works again when Survivors of the Void is disabled
Drifter's passive works when Alloyed Collective is disabled in character select
Seeker's Reprieve and Volcanic Egg no longer get you out of bounds
False Son's Laser of the Father now scales with attack speed; the hidden invincibility on his dash was replaced with a small armour boost; his 1% crit chance (a deliberate easter egg) is now 0%
CHEF's jump is smoother and Roll no longer misbehaves when you jump or land mid-skill
Engineer's turrets now receive Temporary Items
An infinite proc chain between Runic Lens and ATG Missile Mk. 1 was removed
Longstanding Solitude turns into a Pearl at level cap, fixing the Gilded Coast softlock
Fixed duplicated, floating or underground spawn nodes on many stages, including air nodes near the Pillars of Mass on Commencement and jump nodes on Prime Meridian and Siren's Call
Fixed bullets and projectiles passing through Adaptive Chests, Cleansing Pools and Shrines of Shaping
Solutional Haunt plays its music again after you beat Solus Wing

What the expansion on top of this patch actually contains.

Which survivors gained a new skill in 1.21.