I use macOS High Sierra 10.13.6.

Safari 12 now disables unsafe extensions. When will the Zotero plugin be updated? Thanks.

Is there a workaround in the meantime?

According to:
https://developer.apple.com/safari/whats-new/
(accessed today, 2018-Sep-18)

# What’s New in Safari

## Safari 12

Safari 12 ships with iOS 12 and macOS 10.14. It is also available for macOS 10.13.6 and 10.12.6.

[...snip...]

## Safari Extensions

### Deprecated .safariextz-style Safari Extensions

Support for .safariextz-style Safari Extensions installed from the Safari Extensions Gallery is deprecated with Safari 12 on macOS. Submissions to the Safari Extensions Gallery will no longer be accepted after December 2018. Developers are encouraged to transition to Safari App Extensions.

### Disabled canLoad Safari Extensions on first launch

Safari Extensions that make use of the canLoad event will be disabled on first launch and users will be notified with a warning that their use is harmful to performance. These extensions can be enabled in Safari Extensions preferences. Developers are encouraged to move to Content Blocker Extensions.

### Removed Support for Developer-signed .safariextz Safari Extensions

Support for developer-signed .safariextz Safari Extensions in Safari 12 on macOS has been removed. They no longer appear in Safari preferences and cannot be enabled. On first launch users will receive a warning notification and these extension will not load.