See Word Processor Plugin Installation for compatibility information. (Note that, while the same system requirements apply, Zotero Standalone users do not need to install the word processor plugins separately.)
1)
Users of earlier macOS versions can install Zotero 4.0.29.11, but various functionality (e.g., syncing) will cease to work in the latter half of 2017 (more info).
2)
Users of earlier macOS versions will be offered an older version of the Safari connector with more limited functionality.