Adventuring
If I'm hidden and a creature with Blindsight or Truesight sees me, am I still hidden?. No. Being hidden is a game state that gives you the Invisible condition. If a creature finds you, you're no longer hidden and lose that condition, as explained in the Hide action (see appendix C of the Player's Handbook).
The Stunned condition no longer prevents you from moving. Is that change intentional?. Yes. We have intentionally increased the differences between the Stunned and Paralyzed conditions, with Stunned now allowing you to move.
Does all magical Darkness block Darkvision?. Magical Darkness blocks Darkvision only if the rules text for a particular instance of Darkness says it does. For example, the Darkness spell specifies that Darkvision can't see through it. That obstruction is a feature of the spell, not a feature of magical Darkness in general.
Is there a hard limit on how many Short Rests characters can take in a day, or is this purely up to the DM to decide?. The only hard limit on the number of Short Rests you can take is the number of hours in a day. In practice, you're also limited by time pressures in the story and foes interrupting.
Are extradimensional spaces, such as a demiplane or the space created by Rope Trick, considered to be on a different plane of existence?. An extradimensional space (aka an extraplanar space) is outside other planes. Therefore, if you're on the Material Plane and your foe is in an extradimensional space, the two of you aren't on the same plane of existence. (See chapter 6 of the Dungeon Master's Guide.)
How do I tell if something in the game is magical?. Game text explicitly states if an effect is magical. Effects created by spells and magic items are always magical. See "Magical Effect" in appendix C of the Player's Handbook.
What's an example of an effect that can reduce Exhaustion?. The Greater Restoration spell can reduce Exhaustion.
What happens after level 20? Is there a cap to character levels?. The highest level a character can reach is level 20. The "Bonus Feats at Level 20" sidebar in chapter 2 of the Player's Handbook provides some ideas for how DMs can use feats as a form of advancements for level 20 characters.
The Frightened condition says "while the source of its fear is within line of sight." Does that mean you have Disadvantage on attack rolls and ability checks even if the source is imperceptible but you have a clear line to its space?. No. If you can't see something, it's not within your line of sight. Speaking of "line of sight," the game uses the English meaning of the term, which has no special meaning in the rules.