Are effects that activate “on a 20” by definition “critical effects”?

Question

In the recently released (November 21, 2016) Unearthed Arana article, Cleric: Divine Domains, the Grave Domain ability Sentinel at Death’s Door has the following effect:

Any effects triggered by a critical hit are canceled.

My question is, are all effects that are triggered on a 20 of an attack roll considered critical effects? Or only effects that are called out as when a critical is made?

Consider the Sword of Sharpness (DMG, p.206):

When you attack a creature with this weapon and roll a 20 on the attack roll, that target takes an extra 14 slashing damage.

Answer 1

No, a 'Critical' effect is not the same as a Natural 20

This is backed up by This Answer which helps differentiate between rolling a 20 on a d20 and landing a Critical.

Take the Sword of Sharpness or Vorpal weapons, for instance. For these magical weapons to activate you must roll a 20 on a d20 , which is not the same as landing a critical hit. This is farther proven when you include the Fighter's Improved Critical ability which allows them to land a Critical on a roll of 18, 19, or 20. In this case if a fighter landed a Critical with a roll of 18 with a Sword of Sharpness, all of the rules for a critical hit would apply; only the extra slashing damage would NOT apply as the fighter did not, in fact, roll a 20; he only rolled an 18.

Sentinel at Death's Door would only save you from effects that happen as a result of a critical hit; such as the Brutal Critical Barbarian feature or the extra dice included normally in a critical hit; not effects the require a natural 20. The wording here matters for certain features, abilities, and magical effects.

Keep in mind that attack rolls (spell or weapon attacks) automatically crit on a roll of 20; but dispite the connection this is NOT the same as a feature, ability, or magical effect that requires a flat 20 to be rolled as proven above when criticals are possible without rolling a 20.

Answer 2

No -- they'd have used "critical hit" if they meant it

My argument is that "no, a natural 20 triggered effect is different from a critical hit triggered effect" -- this is because something could be immune to critical hits. Historically, this has been true of undead, elementals, and constructs; while the stock 5e Monster Manual does not use this option, the wording you see here appears to be designed to leave that possibility open for the future: the special ability of the weapon would still trigger in that case, but the critical hit does not.

Furthermore, Champions have an expanded crit range with the Improved Critical class feature, which means that effects that trigger on a 20 stay at a flat 5% instead of improving to a 10% chance in a Champion's hands. They also don't activate when hitting a paralyzed or unconscious opponent, which otherwise automatically grants the critical hit if in close combat.