The styleguide only indicates that the word Pipeline (in the context of Jenkins Pipeline) should be title-cased (with a capital "P"). This guide also states that titles/headings should use sentence case.
We have lots of instances of the words 'Declarative' and 'Scripted' in the context of Pipeline being written in title-case, which also gives the impression that these words are proper nouns.
However, it's probably a bit of a stretch to consider these words as proper nouns (and doing so impairs their readability somewhat - especially when these words are used numerous times in a sentence). Therefore, I'd like to propose converting all relevant instances of these words in the Jenkins User Documentation (and any other related documentation) to lower-case words (naturally with the exception of these words being used at the start of headings and sentences).