Are there any other rule mechanics that could grant Thieves' Cant?

Question

Thieves' Cant is a class feature of Rogues in 5e.

As it is a method of communication, can it be taken as a language? If not, is it mechanically equivalent to something that can be swapped so that characters of another class can use it without multiclassing?

Are there any other rule mechanics e.g. background, that could grant Thieves' Cant?

Answer 1

A few, but they all explicitly require DM's permission

As it is a method of communication, can it be taken as a language?

Not as a part of standart character generation. You will require an explicit DM's permission:

With your DM's permission, you can instead choose a language from the Exotic Languages table or a secret language, such as thieves’ cant or the tongue of druids.

Thieves' Cant is meant to be granted by a unique class feature. Even spells like Comprehend Languages do not allow you to understand Thieves' Cant.

Are there any other rule mechanics e.g. background, that could grant Thieves' Cant?

There are no "official" ways to get this feature other than taking a level in Rogue. However, with the DM's approval, you can get it from a custom background, homebrewed class archetype, or learn as a language.

Lore-wise, Thieves' Cant is anti-language or secret language is the jargon or language of a group, often employed to exclude or mislead people outside the group. It is a skill to hide secret messages in whatever language you are currently speaking.

Rules as written, a DM can allow Thieves' Cant as a custom background feature. See the PHB page 125 "Customizing a background":

If you can't find a feature that matches your desired background, work with your DM to create one

You can also learn Thieves' Cant during your downtime, if there's someone who agrees to teach you:

You can spend time between adventures learning a new language or training with a set of tools. Your DM might allow additional training options.

First, you must find an instructor willing to teach you. The DM determines how long it takes, and whether one or more ability checks are required.

Answer 2

It's a "secret language"

The rules divide languages into three groups: Standard, Exotic, and Secret. In the section on Languages about what you can pick as a language, the rules say:

Choose your languages from the Standard Languages table, or choose one that is common in your campaign. With your DM’s permission, you can instead choose a language from the Exotic Languages table or a secret language, such as thieves’ cant or the tongue of druids.

That describes thieves' cant as a "secret language", which (if the DM approves doing so) you could take like any other language given by your background, or learn like researching any other language.

I could certainly understand a DM not approving doing so, given how different thieves' cant is from other languages. But you're specifically asking about how the rules handle it mechanically, and from that Languages section I think it's pretty clear that from a rules perspective it's a language like any other.

Answer 3

Thieves' Cant is not mechanically equivalent to anything that can be swapped out, however...

True polymorph or Shapechange

The following monsters have Thieves' Cant listed for them:

1. Assassin (MM)
2. Deep Scion (Volo's)
3. Master Thief (Volo's)

If you True polymorph or Shapechange into these creatures, you should be able to use Thieves' Cant.

Ring of Mind Shielding

Here's an alternate solution that can be done without access to lvl 9 spells, albeit with a few problems...

1. Find an evil-aligned rogue
2. Slip the ring on his finger while he's sleeping
3. Murder him. Hopefully he won't want to pass on to the afterlife due to his evil alignment. Thus his soul will be trapped in the ring.
4. Wear the ring
5. Whenever you suspect you hear Thieves' Cant, ask the soul in the ring what it means

Of course there are a number of problems with this method.

1. The evil rogue might lie to you
2. You can not always identify whether Thieves' Cant is present
3. Imperfectly communicating to the rogue what you see/hear
4. Someone stealing the ring
5. The rogue passing on to the afterlife anyway

You may be able to get around problem 2 and 3 if you can convince your DM that the Ring of Mind Shielding is cooler if the soul inside can see and hear what you can see and hear.