There are no specific guarantees offered to users about the availability of old binary versions of a specific flatpak. Whilst there’s some older versions offered the ostree repo is regularly pruned of old versions. If you have a requirement for a specific older version of a flatpak that’s not available as a binary then I’d encourage you to check out its manifest from github and build it locally. Whilst we’re not quite able to make guarantees of absolute reproducibility of builds the flathub guidelines and the properties of flatpak mean that any user should be able to recreate any version they want with a couple of commands and a bit of CPU time (modulo the files in the manifest being available and any changes to runtimes etc.)
Flathub is a service offered to make the lives of Linux Application Developers easier and as such takes a slightly different point of view to other ways to get applications on Linux. We want to make sure that bug fixes from the developer are delivered to users and that they’re not reporting old fixed bugs again and again and that the configuration everyone uses is as similar as possible.