Humility says "all creatures lose all abilities and have base power and toughness 1/1" and Endless One comes into play with X +1/+1 counters.
According to a comment in How to understand Humility once and for all, triggered abilities don't get triggered with Humility, but I'm thinking Endless One's ETB "ability" doesn't count because it's not a triggered ability? Because it doesn't say "when it enters the battlefield X +1/+1 counters are placed on it"?
As of the Ixalan rules update, Endless One will not enter the battlefield with counters if Humility is on the battlefield. As other answers have mentioned, Endless One's ability is a replacement effect that modifies how it enters the battlefield. The new version of rule 614.12 says:
Some replacement effects modify how a permanent enters the battlefield. (See rules 614.1c–d.) Such effects may come from the permanent itself if they affect only that permanent (as opposed to a general subset of permanents that includes it). They may also come from other sources. To determine which replacement effects apply and how they apply, check the characteristics of the permanent as it would exist on the battlefield, taking into account replacement effects that have already modified how it enters the battlefield (see rule 616.1), continuous effects from the permanent’s own static abilities that would apply to it once it’s on the battlefield, and continuous effects that already exist and would apply to the permanent.
Applying this kind of replacement effect involves checking continuous effects that already exist. Humility's ability is one such continuous effect, and applying it removes the ability that would add the counters.
The Ixalan Comprehensive Rules Update goes into some detail about how this rule changed, and it specifically mentions a similar situation:
Humility causes creatures with modular to enter without their +1/+1 counters.
For reference, the definition of Modular is
Modular represents both a static ability and a triggered ability. “Modular N” means “This permanent enters the battlefield with N +1/+1 counters on it” and “When this permanent is put into a graveyard from the battlefield, you may put a +1/+1 counter on target artifact creature for each +1/+1 counter on this permanent.”
The part of the Modular ability that affects how the permanent enters the battlefield has basically the same wording as Endless One's ability, so Endless One's ability is affected the same way by the rules change.
You are right that it doesn't affect it because of the below rule, it will get it's counters and it is indeed not a triggered ability:
614.12. Some replacement effects modify how a permanent enters the battlefield. (See rules 614.1c–d.) Such effects may come from the permanent itself if they affect only that permanent (as opposed to a general subset of permanents that includes it). They may also come from other sources. To determine which replacement effects apply and how they apply, check the characteristics of the permanent as it would exist on the battlefield, taking into account replacement effects that have already modified how it enters the battlefield (see rule 616.1), continuous effects generated by the resolution of spells or abilities that changed the permanent’s characteristics on the stack (see rule 400.7a), and continuous effects from the permanent’s own static abilities, but ignoring continuous effects from any other source that would affect it.
Endless One's ability is not a triggered ability.
112.3c Triggered abilities have a trigger condition and an effect. They are written as “[Trigger condition], [effect],” and include (and usually begin with) the word “when,” “whenever,” or “at.” [...]
Endless One's ability is a replacement effect.
614.1c Effects that read “[This permanent] enters the battlefield with . . . ,” “As [this permanent] enters the battlefield . . . ,” or “[This permanent] enters the battlefield as . . . ” are replacement effects.
Because Endless One's ability is a replacement effect, it ignores continuous effects that are not its own for the purposes of determining how the effect would apply.
614.12. [...] To determine which replacement effects apply and how they apply, check the characteristics of the permanent as it would exist on the battlefield, taking into account replacement effects that have already modified how it enters the battlefield (see rule 616.1), continuous effects generated by the resolution of spells or abilities that changed the permanent’s characteristics on the stack (see rule 400.7a), and continuous effects from the permanent’s own static abilities, but ignoring continuous effects from any other source that would affect it.
Although Humility's continuous effect is ignored momentarily, it still applies as the permanent enters. For example:
The Endless one will be a 1/1, the ETB trigger will not trigger as it never had them. Any replacement effects will be bypassed as they never exsisted.
Rulings sited here.
It really comes down to triggers. That's it. Enters with blank is not a trigger. "When" it enters is. Modular affects the card "as" it enters and then is lost as it hits the field. Still gets the +1+1 but is a 1/1 and now is a 2/2. (A 1/1 with a +1/+1 counter).
611.1. A continuous effect modifies characteristics of objects, modifies control of objects, or affects players or the rules of the game, for a fixed or indefinite period.
611.3c Continuous effects that modify characteristics of permanents do so simultaneously with the permanent entering the battlefield. They don’t wait until the permanent is on the battlefield and then change it. Because such effects apply as the permanent enters the battlefield, they are applied before determining whether the permanent will cause an ability to trigger when it enters the battlefield.
Example: A permanent with the static ability “All white creatures get +1/+1” is on the battlefield. A creature spell that would normally create a 1/1 white creature instead creates a 2/2 white creature. The creature doesn’t enter the battlefield as 1/1 and then change to 2/2.
Humility changes "the rules of the game". It affects all creatures no matter where they are (In play, in your hand, graveyard). If Humility is in play, a creature in your hand has no abilities and is a 1/1.
So, if Humility is in play, Endless One (and Hangarback Walker for that matter) has no abilities. You can pay {0} and have a 1/1 creature. If Endless One is already in play when Humility hits the battlefield, Endless One keeps its +1/+1 counters and its base P/T becomes 1/1 instead of 0/0.
