Many of the variant rules from the 2014 Core Rulebooks are not present now.
The Monster Manual hasn't been released yet, so for now, these are the variant or optional rules in the 2024 PHB and DMG:
PHB:
- Playing on a Grid (Chapter 1: Playing the Game, p. 25)
- Variant: Equipment Sizes (Chapter 6: Equipment, p. 219)
DMG:
- Miniatures (Chapter 2: Running the Game, p.44). This section expands the rules in the PHB for depicting combat with miniature figures on a grid.
- Variant: Training to Gain Levels (Chapter 5: Creating Campaigns, p. 49)
- Loyalty (Chapter 3: DM's Toolbox, p. 89)
- Renown (Chapter 3: DM's Toolbox, p. 92)