Why can Sailor Pluto be away from the Space-Time Door? In the classic 90s anime, Sailor Pluto was commissioned by Queen Serenity (mom of Princess Serenity [Sailor Moon's ancient incarnation]) to guard the Space-Time Door, which she is shown doing in the R season, but she is later allowed to leave, and thus shows up with a civilian form (Setsuna) in the S season.  One of the 3 rules given by Queen Serenity was that she could not leave her post at the door.  She does go on to break all of the rules, but according to Wikipedia, Neo-Queen Serenity (future Sailor Moon) gives her permission to leave her post.

But why?  Does the door not need to be guarded anymore?  Is it temporary leave and she will need to return eventually, or is it permanent leave?

In the manga, Sailor Pluto's guarding location might be located outside of time (or is it located inside of the palace in 30th century Crystal Tokyo?), so perhaps she could technically be there as well as have a 20th century civilian incarnation that is at a point inside of time, so she could apparently continue to guard the door while she shows up in S, the SuperS movie, and Sailor Stars (but this is unrelated to why Neo Queen Serenity would excuse her from her post in the classic anime).

In the Sailor Moon Crystal canon, is Sailor Pluto able to and/or permitted to leave her post at the door? Is this aspect identical to the manga version?
In the Crystal continuity it is unknown as it is a work in progress, and did not progress too much further from Pluto's introduction, so it is unknown for Crystal continuity if she will leave the door or not. There is still no canon material on the matter.

As to why she can leave the door, she has the power to seal off entire worlds out of the fourth dimension (the time dimension)(ref).

Since you need a time-space key to safely navigate fouth dimension, she could secure all keys, lock the access to the place where the keys were stored and leave for a short time. 

Probably her leave cannot be defined as permanent or temporary. Since the fourth dimension is outside normal time, she can safely exists in the fourth dimension and outside it:


  However, for all intents and purposes, in the manga the technicality of Sailor Pluto existing outside of time means she is able to be reincarnated as a normal woman while still existing back at her post at the Time Gate.(ref)


Sailor Pluto's racial status is also subject of controversy. While Endymion/Tuxedo Mask is from the Earth realm, Usagi/Sailor Moon is from the Silver Millenium race, the other sailor senshi seem to be reincarnated humans, Sailor Pluto is more of a goddess than a human:


  She is also stated to be the daughter of Chronos, god of time. (manga act 18, original anime ep. 82).


They made significant changes in Crystal from the original Sailor Moon works (like giving Tuxedo Mask an attack), so I wouldn't expect it to be identical, but to follow closely.

You can compare Crystal and the original making a parallel between the original Yamato series and Yamato 2099 that aired in 2014. They tend to follow the original work closely, but making adaptations and concessions to suit XXI century's scientific and social developments.