Unfortunately, Apple prohibits App Store apps from reading or modifying keywords, titles, descriptions, and other user entered metadata. Some apps have made their own keyword systems, but they cannot store the data into the photo library, which means they don’t sync over iCloud, etc.
I plan to add keywords to the Finder-side of things because that doesn’t have that restriction / limitation.
If Apple ever decides to giver App Store apps this ability, then I would add the feature.
By the way, there is a roadmap on the website: https://www.gentlemencoders.com/nitro-roadmap/index.html