It is not possible to create Flow and make it available for whole tenant to be used in any library/list in SharePoint site.
As there is possible to pick Flow from public gallery. It will be nice to have something similar on tenant level as mentioned in previous post here Reusing flows post.
It will be nice to create something that will be called Organization gallery so the Flow will be findable here for any user shared with.
It's now possible to reuse copies of flows inside of your organization: https://flow.microsoft.com/blog/send-a-flow-as-a-copy/
@StephenThat link isn't working for me... 404 error. Current link that is: https://flow.microsoft.com/en-us/blog/send-a-flow-as-a-copy/ (has the "en-us" in there so I'm not sure what it'll do with other locals).
I'd say not completed @Stephen. Imagine you change a Flow, you still need to change all copies. So a poor man's workaround is more what this is - and still useful for a few scenarios. But for large rollout, no.