As of the current version, Factoriopedia (the in-game encyclopedia) shows everything from the get-go, with no option to hide un-researched things like Recipe Book and FNEI could do. All items, all entities, all enemies.

In addition, there are menu simulations (the animated backgrounds) for every planet. If you were hoping to discover planets on your own, it's very difficult to avoid learning about them ahead of time, unless you disable menu simulations and don't use Factoriopedia.

This mod hides menu simulations and Factoriopedia pages from other planets, and lets you un-hide them one-by-one when you want to in the mod's settings.

Can the mod automatically reveal things when appropriate?

Unfortunately, due to the way this hiding works, it cannot automatically detect the state of your game and you have to change the mod settings in the Mod Settings menu, under the "startup" tab. There will be a chat reminder message the first time you reach any new location.

Remember that "startup" mod settings can only be changed from the main menu. You CAN change them for an existing game, you just need to save and quit to the menu first.

Can I hide the content anywhere it might appear, not just hiding the Factoriopedia pages?

Yes! Please uncheck the "Only hide in Factoriopedia" mod setting.
This will hide content from any selection UI (toolbar selection, logistic requests, circuitry signal, etc.) as well as from technology effects. Some technologies might look like they have no effect.

Can the mod also hide technologies from the tech tree?

No, but Discovery Tree is also available for 2.0 and will probably do what you want!