This Mod been folded into the Train's Tweaks mod under the Experience feature for 1.21+.

LinearLevels adds ~~three~~ five configuration options for how you want the experience "curve" to work in your game. These configuration options can be found in ModMenu or similar mods. Requires Cloth Config API.

By default, every level will take 10 experience points (compared to vanilla this feels quite low so I recommend configuring it to your liking).

Alternatively, you can enable Curve Mode, which uses the formula (XPToNextLevel = (baseXPForOneLevel + (experienceLevel * curveModeMultiplier)). I find Curve Mode's defaults to be my preference, as the curve doesn't get ridiculous after level 30 like in vanilla.

(yes, the name Linear Levels isn't accurate in this case, I know)

For the exact numbers of both modes and their comparisons to vanilla on a graph, see this Google Sheet.

1.1.0 adds an additional config option: Level Cap. It's disabled by default, but by setting a value, the experience required for the next level will stop increasing at the specified level.

FAQ:

How will this work with XP storage mods?

It depends on how those mods work. In other words, I don't know. The only thing this mod does is mixin to one method in the player entity.