should theme/demo1 branch needed to deploy to prod first before merging the
the,e/demo2 in master?

After all, creating a tag is near-zero effort, so why not! 🙂

For stuff I push up to GitHub (plugins and themes, specifically), I’ll always use tags. The benefit there is it will automatically create a release archive for the project (e.g. https://github.com/jazzsequence/book-review-library/releases). For client work, obviously it’s a different thing, but we’ve had issues where we’ve needed to restore the production site back to a previous commit, and IMO finding the right tag is easier than digging through the commit history.

On the other hand, I tend to move my tags around a lot and delete/readd them when I end up finding a problem that needs to be fixed in that version, so I totally empathize with not doing it. 🙂

For WordPress, I don’t really use tags, since the version number is already “hardcoded” in the plugin or theme. I normally commit with the version number as commit message right before release. That’s my tag I guess.