For instance, rolling a Cure Wounds with a negative WIS modifier can produce a negative heal amount.  https://www.sageadvice.eu/can-cure-wounds-cast-with-a-negative-spell-casting-moodier-potentially-do-damage-to-a-friendly/ seems to imply that the intention is that there is a floor at 1 point of healing, regardless of spell modifier.

Negative modifier may or may not be in effect depending if the game element gives a minimum value or if it's specifically ignored like Dexterity for Heavy armor or Constitution for how long one can go without food. 

Things like healing or damage may not be inferior to 0. 

Anything that guarantees a base minimum of 1, regardless of modifier specifies that. For example, from the entry on the “Spellcasting” feature for Clerics:

Preparing and Casting Spells

The Cleric table shows how many spell slots you have to cast your cleric spells of 1st level and higher. To cast one of these spells, you must expend a slot of the spell’s level or higher. You regain all expended spell slots when you finish a long rest.

You prepare the list of cleric spells that are available for you to cast, choosing from the cleric spell list. When you do so, choose a number of cleric spells equal to your Wisdom modifier + your cleric level (minimum of one spell). The spells must be of a level for which you have spell slots.

For example, if you are a 3rd-level cleric, you have four 1st-level and two 2nd-level spell slots. With a Wisdom of 16, your list of prepared spells can include six spells of 1st or 2nd level, in any combination. If you prepare the 1st-level spell cure wounds, you can cast it using a 1st-level or 2nd-level slot. Casting the spell doesn’t remove it from your list of prepared spells.

You can change your list of prepared spells when you finish a long rest. Preparing a new list of cleric spells requires time spent in prayer and meditation: at least 1 minute per spell level for each spell on your list.

Restoring negative hit points isn't necessarily the same thing as dealing damage. And dealing negative damage isn't necessarily the same thing as healing. Both things are basically undefined in the rules. There's no guidance for how to resolve them. Though realistically I hardly think it'll ever come up.

I would rule that the minimum is 0.

Quote from ChoirOfFire >>

Restoring negative hit points isn't necessarily the same thing as dealing damage. And dealing negative damage isn't necessarily the same thing as healing. Both things are basically undefined in the rules. There's no guidance for how to resolve them. Though realistically I hardly think it'll ever come up.

I would rule that the minimum is 0.

 
Actually, the rules do explicitly state that a negative result = 0.

(https://www.dndbeyond.com/sources/basic-rules/combat#DamageRolls)

Damage Rolls

Each weapon, spell, and harmful monster ability specifies the damage it deals. You roll the damage die or dice, add any modifiers, and apply the damage to your target. Magic weapons, special abilities, and other factors can grant a bonus to damage.

With a penalty, it is possible to deal 0 damage, but never negative damage.

When attacking with a weapon, you add your ability modifier--the same modifier used for the attack roll--to the damage. A spell tells you which dice to roll for damage and whether to add any modifiers.

If a spell or other effect deals damage to more than one target at the same time, roll the damage once for all of them. For example, when a wizard casts fireball or a cleric casts flame strike, the spell's damage is rolled once for all creatures caught in the blast.

While the rules for healing do not explicitly state the same, since both are covered under the same “Damage and Healing” section of Chapter 9: Combat, I would expect that RAI it applies to both.

Good find!

Sorry for boarding this old ship, but I just created a Paladin with a negative Charisma modifier (intentionally), however, I just realized, that Heroism gives a touched creature tempory hitpoints based on the spellcasting ability modifier, which RAW it would give -2 temporary hitpoints... so... What does this spell do now? Simply makes that creature immune to frightend and that's it?

It's probably the only spell I would cast, which uses the spell-modifier.

Quote from BurningFalcon >>

Sorry for boarding this old ship, but I just created a Paladin with a negative Charisma modifier (intentionally), however, I just realized, that Heroism gives a touched creature tempory hitpoints based on the spellcasting ability modifier, which RAW it would give -2 temporary hitpoints... so... What does this spell do now? Simply makes that creature immune to frightend and that's it?

It's probably the only spell I would cast, which uses the spell-modifier.

Correct, all it would do is make that creature immune to being frightened. Even if the creature already had THP, it wouldn’t remove 2 of them or anything. As per the rules for THP:

Temporary Hit Points https://www.dndbeyond.com/sources/basic-rules/combat#TemporaryHitPoints

Some spells and special abilities confer temporary hit points to a creature. Temporary hit points aren't actual hit points; they are a buffer against damage, a pool of hit points that protect you from injury.

When you have temporary hit points and take damage, the temporary hit points are lost first, and any leftover damage carries over to your normal hit points. For example, if you have 5 temporary hit points and take 7 damage, you lose the temporary hit points and then take 2 damage.

Because temporary hit points are separate from your actual hit points, they can exceed your hit point maximum. A character can, therefore, be at full hit points and receive temporary hit points.

Healing can't restore temporary hit points, and they can't be added together. If you have temporary hit points and receive more of them, you decide whether to keep the ones you have or to gain the new ones. For example, if a spell grants you 12 temporary hit points when you already have 10, you can have 12 or 10, not 22.

If you have 0 hit points, receiving temporary hit points doesn't restore you to consciousness or stabilize you. They can still absorb damage directed at you while you're in that state, but only true healing can save you.
Unless a feature that grants you temporary hit points has a duration, they last until they're depleted or you finish a long rest.

The -2 THP would technically count as a separate “pool” of THP that player could choose. So if they had 12 THP, and you cast heroism on them, then they would get to choose between the pool of 12 they already had vs to pool of -2 you’d be offering them. Who would choose the -2?!? Exactly.