The mod had a rebalance.
You kill an enemy, you get experience for it, you always need the same amount of experience, experience does not depend on the weapon, while you are going through the game the enemies become more difficult, and their number does not change, not counting the appearance of potions and a water candle. As a result, you can kill enemies with absolutely any weapon, getting the same level at all stages of progress in the same time, and due to the fact that you always need the same experience, you have no reason to take a new weapon saying "well, damn, here you need 10,000 experience and here 1000, I'll take the right one to improve it already"

To understand, I'll tell you how it was before
I think the starting was 1000/1250, and the increase per level was 250/50, I don't remember exactly, but the numbers went in pairs, left to left, right to right
There was no experience for killing, experience was for dealing damage and for the amount of damage. As a result, the more damage you dealt and the faster you attacked, the faster the improvement was. Rapid-fire weapons were always better, damaging weapons were always better, and when you received an improvement, you needed more experience. What was the result?
You take a fast/high damage weapon, the strongest you have, upgrade it to a point where the experience requirement becomes too high, then swap it for a new strong weapon that has roughly the same stats as your upgraded one, but will upgrade faster because it requires 1000 exp instead of 10,000.
At the beginning of the game, weapons received almost no upgrades, a couple of levels at most, when by the end of Calamity you could get 100 levels without any problems. Only base damage was increased, so the starting weapon received a large increase, when the end-game weapon barely felt any change in damage. However, the end-game weapon felt an increase in other stats, such as crit chance, attack speed, and more.
Now, for example, the increase is not only in base damage, but also in percentage. Therefore, for level 100 you will have at least 200 damage, 100 base and +100% on top, which significantly strengthens even those weapons where the damage is 20,000 damage and higher. At the same time, everything is the same in sensations as at the very beginning of the game.

Previously, it all came down to the fact that the player changed the most effective weapons. The mod was not so badly balanced, and played normally, but compared to what it is now, it had a terrible balance. As an example, you could just endlessly try to kill the boss until your weapon began to kill him in a few seconds. Now you have to try and intentionally play with the weapon so that it becomes good. Now there are no strange abuses, and the most effective thing you have is the blood moon.
To prevent progress from being slow, all stats began to give 2 times more per level, and if even this seems slow, you can simply adjust the experience multiplier.
Now the mod does not break the balance by itself, and it is much more interesting to play with it, and if you want to do everything that is on the screenshots - it is enough to do, for example, x20 experience, and in minutes you will receive weapons that can kill final bosses in seconds