I found a few, but they are all bad, like true polymorph to improve ability scores (but I lose class features), does someone know any way to improve ability scores without losing class features, and without using magical items, only spells and class shenanigans?
Temporary and permanent increases are welcome, my objective is improve warlock damage from lifedrinker.
At 20th level, a Barbarian gains +4 to strength and +4 to constitution as their capstone ability. This meets your requirement of increasing ability scores without items, but is going to prove unlikely to help with increasing a Warlocks damage from lifedrinker.
There is the option of casting the 9th level spell Shapechange, which allows you to change your strength, dexterity and constitution, but once again, this won't help you with your specific problem, because you keep your own charisma. It's also a 9th level Wizard / Druid spell, so it seems unlikely you'll have access to it as a Warlock.
For that one, you're most likely out of luck. D&D 5E has done away with almost all effects that increase or decrease ability scores, to streamline the game and to make it so you won't have to recalculate all your skill and attack bonuses on the fly.
There are still a few monsters that can drain ability scores (the Intellect Devourer and Shadow come to mind), but as far as I can find, there are no official spells or abilities that increase ability scores asides from the Barbarian capstone. Those kinds of things have all been replaced with the advantage mechanic.
PS:
On a pure technicality level, Ability Score Increases you get at certain levels are also technically a class feature, so those are also an option, as pointed out by @DaleM, but I assumed you were already aware of those.
The shapechange spell states:
You assume the form of a different creature for the duration. The new form can be of any creature with a challenge rating equal to your level or lower. The creature can't be a construct or an undead, and you must have seen the sort of creature at least once. You transform into an average example of that creature, one without any class levels or the Spellcasting trait.
Your game statistics are replaced by the statistics of the chosen creature, though you retain your alignment and Intelligence, Wisdom, and Charisma scores [...]
You retain the benefit of any features from your class, race, or other source and can use them, provided that your new form is physically capable of doing so. You can't use any special senses you have (for example, darkvision) unless your new form also has that sense. You can only speak if the creature can normally speak. [...]
This allows you to change into another creature, changing your Strength, Dexterity, and Constitution scores, while keeping your class features.
