I'd like to have a way to have a master slide that will be used in multiple keynote presentations.
I don't want to have to manually open the master, copy, open the target, and paste.
Ideally, a link from a slide would update the target slide deck automatically.
Short answer: You can't.
Keynote supports the concept of templates, that define an underlying 'look and feel' for a presentation.
Keynote also supports 'Master slides' that have layouts and formats for common presentation slides such as Title, Agenda, Bullet List, etc.
When you create a new Keynote Presentation from a template, those Master slides are, essentially, copied into the presentation, but are subsequently disconnected from the master template.
Changes made to the Master Slides within a presentation will apply to all slides that use that Master (e.g. if you change the 'Agenda' Master Slide, then all 'Agenda' slides in the presentation will reflect that change).
However, since this presentation is disconnected from the master template, it doesn't care (or know) about any changes to the template.
In other words, changes made to the template documents will affect all future presentations that are based on that template, but will not affect existing presentations that were based on that template.
TL;DR: there's no built-in way to make changes to a given slide format across multiple documents.
Changes to Templates affect future presentations.
Changes to Master Slides affect the current presentation only.
You're welcome to suggest this as a feature request via Feedback - Keynote - Apple but you have to consider how this could (or could not) work across shared systems (e.g. if you send a presentation to another user, should the slide use their version of the Master Template, or yours? How would they know there's been a change?
Is it possible to have one slide in a keynote linked to another keynote slide (single update for both)?
