I'm trying to figure out how to add cantips to a Cleric subclass, without those cantrips being automatically learned—i.e. just so that they become optional to learn for a cleric of that subclass. If the solution results in the cantrips automatically being learned, that's not what I'm trying to do.

Anyone know how to accomplish this?

When you're editing a subclass feature, there's a section at the bottom called Additional Spell List. You can add the cantrips there, and then the subclass will be able to prepare those spells via Manage Spells.

Alas, as with Domain Spells, anything put in there counts as automatically prepared/learned.
With cantrips, using the additional specific spells bar, they're automatically added to my spell list, but aren't added to the list of spells that I can choose in my spell-selection tab.

Or maybe I just need to wait longer for the cache to update... I'm trying a few combos with and without additional specific spells, and adding spells (not counted as known spell) with the feature.

Quote from Jety_Lefr >>

Alas, as with Domain Spells, anything put in there counts as automatically prepared/learned.
With cantrips, using the additional specific spells bar, they're automatically added to my spell list, but aren't added to the list of spells that I can choose in my spell-selection tab.

Or maybe I just need to wait longer for the cache to update... I'm trying a few combos with and without additional specific spells, and adding spells (not counted as known spell) with the feature.

 
Naruhoodie didn’t say to put them in the “Additional Specific Spells” field under Basic Information. They said to add them to a class feature under the “Additional Spells List” form all the way at the bottom.

Thankyou! I definitely didn't notice that. I've never used that modifier before when adding spells, 'cause I've never added spells in quite this way before.
I'll let the cache update and see how it works! 👍

It's a relatively new feature (well, new as in the past year or so), I think added when they were working on Tasha's.

Another useful thing about Additional Spell Lists in homebrew is that you can use it to add spells that aren't Basic Rules/SRD (free) while still being publishable, since it works via Manage Spells, and the spell will just not show up if the homebrew user doesn't have access to the spell's source.

Well that's convenient.

It hath worked!!