When trying to add the spells to a homebrew subclass:

• Armor of Agathys 
• Swift Quiver

i get an error:

> This homebrew Subclass has data mapped to it that is licensed content or private homebrew. This typically occurs when licensed or private homebrew spells are linked to the Subclass.

After having removed these spells, everything is fine...

That is correct. Both of these spells are licensed content, meaning they have to be paid for here. They are contained within the Player's Handbook, not the Basic Rules which are free to use. 

It does make sense that they are indeed licensed content :)

Subclasses from the PHB do get to use such spells though. It really sucks that published homebrew is more limited in options then a vanilla Tempest Domain Cleric :(

That only stops you being able to publish the material. You don't need to do that if you're just intending on using it within a campaign. All homebrew regardless of published status can be shared in a campaign.

Quote from Esmeril >>

It does make sense that they are indeed licensed content :)

 

Subclasses from the PHB do get to use such spells though. It really sucks that published homebrew is more limited in options then a vanilla Tempest Domain Cleric :(

 That’s because WotC owns the legal rights to those spells, and we do not.

WotC gave away free rights to everything published in the SRD, which DDB combined into the Basic Rules for us. So DDB’s “Basic Rules” is really the “Basic Rules & SRD.” WE own the rights to those spells because WotC gave them away for free. Make sense?

 Thanks for the replies,

Quote from IamSposta >>

WotC gave away free rights to everything published in the SRD, which DDB combined into the Basic Rules for us. So DDB’s “Basic Rules” is really the “Basic Rules & SRD.” WE own the rights to those spells because WotC gave them away for free. Make sense?

 Well, if that means that DDB is legally prohibited to publish homebrew referring to these spells, then yes :)

But I doubt that.
The reason being: right now if I buy the XGTE and not the PHB, I will be able to create a Conquest Paladin, referring to Armor of Agathys, a licensed spell that I have not purchased. 

Apparently PHB spells are not off-limits for Wizards when creating new subclasses, why should they be for us homebrewers?

Quote from Emmber >>

All homebrew regardless of published status can be shared in a campaign.

 Yeah, I understand that. It just boggles me that I cannot publish a subclass made according to the same rules WotC adhere to when creating new classes. No biggie though :D

Well WotC and DDB both have the rights to publish using that content. We only have the rights to use the content - if we purchased it.

The problem stems in the fact the Homebrew system does not factor what happens if somebody adds that homebrew but doesn't have access to non-free content mapped to it.

This is being looked into and is planned to be supported in the future.

Just clarifying from some posts: the issue isn't actually licensing permissions, since you are not giving people access to licensed content. It's a homebrew tool system issue because it is not able to correctly handle for such events (homebrew is handled differently to the official stuff). D&D Beyond don't need to obtain any extra rights or permissions or anything from WotC - it's a system issue not a licensing one. At an, as of yet, undetermined point in the future this will likely stop being a problem.

Thank you for the answer, Cyb3rM1nd.

I'm looking forward to this undetermined point in the future with hope :)

Five and half years later and we are still waiting lol.