Are there any readily available options for PCs to penalize enemy saving throws?

Question

There are many ways to get advantage/disadvantage on attacks: the Dodge and Help actions are specifically designed for this in combat, and 10 of the 14 conditions in Appendix A have at least one effect causing (dis)advantage on attacks. Also note they use the universal term "attack roll" with no distinction between melee/ranged or weapon/spell.

By comparison, effects on saving throws seem to be quite rare and limited in scope. In terms of common actions & conditions:

• The Dodge action gives advantage on Dex saves;
• The restrained condition gives disadvantage on Dex saves;
• 4 conditions (paralyzed, petrified, stunned, unconscious) cause Str/Dex saves to fail automatically, in addition to other debilitating effects.

This still leaves 4 of the 6 ability score saves unaccounted for.

What options are readily available for a PC to penalize a specific saving throw? For example, in order to make a spell that requires that save more likely to take effect.

For the purpose of defining "readily available", I'm looking at these criteria:

• Either applies universally to all saves (such as "the next spell you cast that requires a save") or lets the PC choose from more than 1 ability score save.
• Is available to PCs below level 5 (doesn't require dedication to a particular class beyond the first advancement tier).
• Appears in any officially-published book (not Unearthed Arcana or third party).

Answer 1

There are several options:

• Heighten Spell Metamagic
  • Requires three levels of sorcerer, lets you impose disadvantage on any spell's save. Costs three sorcery points.
• Bane spell
  • Requires a level of Bard or Cleric, or Magic Initiate, or something else giving you it or a generic level 1 spell off the Bard or Cleric lists. Doesn't actually impose disadvantage, but rather subtracts 1d4 from attack rolls and saving throws for up to a minute. Requires Concentration. Affects multiple targets. At higher levels affects even more targets.
• Unsettling Words
  • Only available to College of Eloquence bards; again, not disadvantage, but subtracts a Bardic Inspiration die from the target's next saving throw.
• Bestow Curse spell
  • Gives a target disadvantage on any one save for a duration that varies with the slot level used. Requires 5 levels in Bard, Cleric, Wizard or a thing that gives you a third level spell off said lists or otherwise access to Bestow Curse.
• Any of the diseases listed in Contagion
  • While contagion is a 5th level spell, it specifically notes that the diseases it can curse targets with are natural diseases that exist in the world. The diseases can presumably, then, be contracted other ways and part of the effects thereof is having disadvantage on a save (there are 6 listed diseases, one for each ability score).
• Gold Dragon Weakening Gas
  • You'd need to bottle it or something, but this gas gives disadvantage on strength saves for up to a minute as one of its effects.
• Treasure from a Mummy Lord's Lair
  • Anyone who takes this has disadvantage on all saves till the curse is broken. If you carry some around, you can trick people into activating the curse, but you yourself will also be cursed.

There are many more options-- almost every casting class and subclass gets something that lets them force disadvantage on arbitrary saves-- but most of the other ones that are class features require being a higher level. There are also lots more options involving monsters than the two I mention, but most of them involve a lot of work to put together without lugging the monster itself around with you. There are also some magic items, of course.

Answer 2

Yes, Heightened Spell

The sorcerer has the Heightened Spell Metamagic option, from level 3, which enables them to spend 3 Sorcery Points to give an enemy disadvantage on a saving throw.

Heightened Spell

When you cast a spell that forces a creature to make a saving throw to resist its effects, you can spend 3 sorcery points to give one target of the spell disadvantage on its first saving throw made against the spell.

Answer 3

The cantrip Mind Sliver might help.

The Mind Sliver cantrip from Tasha's Cauldron of Everything makes the target subtract 1d4 from its next saving throw:

You drive a disorienting spike of psychic energy into the mind of one creature you can see within range. The target must succeed on an Intelligence saving throw or take 1d6 psychic damage and subtract 1d4 from the next saving throw it makes before the end of your next turn.

The debuff depends on failing the saving throw forced by this spell; it does not impose disadvantage but it reduces (by at least one point) the ST of the target.

Obtaining this spell requires at least one level in the Wizard, Sorcerer or Warlock classes, or, if the DM allows the use of feats, the Magic Initiate feat.