I was looking at Mordenkainen's Tome of Foes and noticed for the Nightwalker info that it said "The plane sucks the life and soul from such audacious [living] creatures" and "Those few who survive do so by sheer luck or by harnessing some rare form of magic".
What kind of magic would that be? Abjuration? Magically-enhanced healing? Immunity to Necrotic damage?
Just as there is no listed rule for exactly what the negative energy plane does to living creatures, the "some rare form of magic" means you as the DM will make up something that fits with your campaign that allows the party to visit the Negative Energy Plane without instantly dying (or, alternatively, you can choose not to do so, making the plane functionally inaccessible).
As an example, in the Exploring Eberron book*, the plane of Mabar (which is a sort of setting-specific mashup of the Shadowfell and Negative Energy Plane) deals 10d6 necrotic damage every minute to any living creature, and if you hit zero HP, you crumble into dust. The rules for this specifically call out that death ward and resistance or immunity to necrotic damage will nullify the effect.
That's not the Negative Energy Plane as such, but it's an example of what you could decide to implement in your game. The defenses could be as simple as those -- making the plane accessible (at least for a short time) using just a few specific spells or items -- or it could be an ancient spell found on a stone slab deep in a mummy's tomb, a magic item that creates a bubble of life energy, or magitech space suits. You decide what makes sense for your game.
That text is basically the game designers lighting up a big neon sign that says PLOT HOOK GOES HERE.
*Exploring Eberron is admittedly a DM's Guild book, but it was written by Keith Baker, the original creator of the Eberron setting. While it isn't "canon" (whatever that means in D&D), it's probably the best insight we have into what the setting is intended to be like in the areas not covered extensively by the main book.
Using true polymorph to transform into a native creature should allow you to survive on the Negative Energy Plane. As a 9th level spell, this would potentially qualify as a "rare form of magic".
According to the spell description, "if you concentrate on this spell for the full duration, the spell lasts until it is dispelled", so the ability to survive would last longer than the hour spell duration.
Fifth edition doesn't seem to have information on what creatures are native to the plane, but nothing in 5e indicates that creatures previously mentioned as native should no longer be.
According to the 3e Manual of the Planes (p.82), certain undead are able to exist on the Negative Energy Plane, including Spectres, Wights, and Wraiths.
Both the 1e (p. 54) and 3e (p. 82 and p. 168) Manual of the Planes mention the Xeg-yi as inhabitants.
Polymorphing into one of these creatures should grant whatever is needed survive.
Nothing—not physical objects, energy, or other spell effects—can pass through the barrier, in or out, though a creature in the sphere can breathe there. The sphere is immune to all damage, and a creature or object inside can’t be damaged by attacks or effects originating from outside, nor can a creature inside the sphere damage anything outside it.
Based on the highlighted sections, as a DM, I would rule that the resilient sphere protects one character from the effects of the Negative Energy Plane.
But it also only lasts one minute. So better move quick and have multiple castings ready.
Of course your DM could also rule that the NEP has the same effect as disintegrate, and destroys the sphere. So better to test it first.
