I agree with enigma_: The need for registrants is not likely to die out for quite awhile. At the same time, this will never be a job for the masses.
I looked at the link in the OP. It's in English, so the original is "organist's assistant".
As may be expected, there is variation in how the terms are used and at least some overlap. Some churches have "organist's assistant" programs for children in grades 3-6/7, in which the selected children are able to sit with the organist in the loft, turn pages, and serve as a registrant (after training). In these instances, a "registrant" is more advanced than an "organist's assistant".
As a job description, however, "organist's assistant" includes not only the duties of registrant but also helping to choose service music, marking hymns in the hymnal, giving the pitch to the cantor, assisting with rehearsal logistics (which involves for most church organists either accompanying or directing the choir), opening and locking both the organ and the loft, offering a critical ear when requested, being a sounding board or consultant on various decisions, and serving as substitute organist whenever the organist is ill or absent for some other reason. It's really an apprenticeship of sorts.
Essential skills for a registrant include:
- ability to follow difficult scores at speed
- ability to sustain concentration, often for extended periods of time
- competence with the page turning
- considering the organist's ability to see and surrounding lighting
- being prepared for unforeseen eventualities* and remaining calm
*For example, a story I read recently was about an organist who allowed some children into the loft during the service. In the middle of a piece, the organ went silent. One of the children had by accident leaned back against the power switch and turned the organ off. Everyone stayed calm, turned the organ back on, and continued. By the next week there was a lock box around the power switch.
Thanks to California81 for directing me to this thread.