While this plant is rare in cultivation it is not endangered or even near threatened according to the IUCN: https://www.iucnredlist.org/species/151848/121570462.
The rarity of this plant in cultivation is likely due to where it grows and the difficulty of growing it. It only grows in Brazil on the banks of the Rio Negro in the Amazon Rainforest. The plant is extremely unique in that wraps its branches around the trunks of trees in what would be a shingling growth habit. It floods quite often where it grows which leads to a really interesting scenario of surviving periods of submersion. Another unique trait as you can see in the photos below is it will turn shades of red and purple in high light conditions.
This plant is best suited to growing in a tropical greenhouse in cultivation. I have managed to get it to grow as a hobbyist in a normal home environment by growing it on top of a bed of sphagnum moss and giving it weekly sink baths. I now keep it on a special custom sphagnum moss pole filled with Hydroton. The moss is kept very loose. The pole is then placed in a large pot with a hole drilled about 1/4 of the way up and filled with more Hydroton to stabilize the pole and allow water to wick upward keeping the air more humid around the plant. The entire pot is given weekly showers.
The dangers of having to keep a plant on the wet side are bacteria and fungus growth. You can see the edges of the clades have started to turn dark and dieback. To prevent this I pour 3% hydrogen peroxide on the plant about once a month.
I do move this plant outdoors when the temperatures are above 60f. Indoors it is grown in a south window, however, I live in an area where the light intensity is lower and has a shorter duration.
A personal role model of mine Margaret Mee is a botanical artist who spent a large part of her life trying to capture this plant in bloom. She was an incredible, beautiful, and extremely brave woman with an amazing life story. There are several books featuring her artwork and a wonderful documentary about her Margaret Mee and the Moon Flower available on Amazon. Here is a preview from youtube: https://youtu.be/7soEbnu6JbM and here is the Amazon Prime documentary: https://www.amazon.com/Margaret-Mee-Moonflower/dp/B0777RFBS5.
In the documentary title Margaret Mee and the Moon Flower, Moon Flower is in reference to the flower of S. wittii as it only blooms at night, and the blooms only last a few hours. This is quite common for many of the species of epiphytic cacti.
Margaret Mee books pictured from left to right:
Flowers of the Amazon Forests: The Botanical Art of Margaret Mee by Margaret Ursula Brown
Margaret Mee’s Amazon: The Diaries of an Artist Explorer by Margaret Mee, Margaret Mee in Search of Flowers of the Amazon Forests by Tony Morrison
The Flowering Amazon: Margaret Mee Paintings from the Royal Botanic Gardens, Kew (Margaret Mee pictured inside jacket photograph by Tony Morrison)
Margaret Mee Return to the Amazon from the Royal Botanic Gardens in Kew (print of a Margaret Mee rendering of S. wittii from the collection of the Library, Royal Botanic Gardens, Kew)