May be this post has some more insights (even if it could be outdated): High availability on Fly.io - #3 by kurt
Only if one is running multi-region Fly apps, does fly deploy --strategy rolling <other-args> prevent outages from client PoV (ref)?
I am compelled to point out that, Machine app instances get fixed 6pn (private) IPs across deploys (unlike regular Fly apps unless they ‘anchor’ themselves to a host), and so, these are likely to not be subject to deploy-induced downtime. If you can, you should move to Machines (after evaluating it for a round or two).