I had no previous experience with having to install and old version in linux: not something you do every day, usually you want to upgrade.
You are right that in windows you don’t allways have easy access to differente versions (depends on the developer) but you can usually find the installer in some place making a search in google, there are plenty of archives that provide downloads.
In Linux I tried a similar approach but no so easy, as it is not a single executable and you need to use the package manager.
No complain about it, but I would like that repositories could have a historical access to packages if you need an old version.
I don’t think this is good idea at all. (or even possible)
If any app (like dt) would require an old version that would mean a complete dependency mess plus security problems.
Also, for darktable we certainly suggest the latest stable version for all users. Even if you don’t care about new features, every version i can remember has a number of old issues bugs fixed (just one example. Search for astronoise & darktable and mostly an improved performance.
There are two problems here. One was already mentioned by @hannoschwalm, the dependency chaos that would result from having different versions with different dependencies. This might in the future more and more be solved by using formats like flatpack or docker.
The second difficulty is the handling of security fixes. Many updates not only include new features (that you may not want) but also security fixes. Then you would either have an insecure system or all security fixes would have to be back ported to all maintained versions.
Your problem results from the wish to use the same database with two different applications (darktable and Ansel). This is in my opinion not the best of ideas. But if you want to go such a special way, it’s not too much to ask to learn compiling. With “normal” use of a Linux system this is not necessary.
That is the usual library versions and depencecies hell.
Windows suffered that for quite long time, with some windows versions being a complete mess to maintain or develope for, as you ended with incompatible demandas of libraries version from differente software packages that used some share dll.
In windows that was worse due to a common central registry.
I had thought it was better in linux, where there was no such central repository and with a package managment that keep track of version depencencies.
If the version of libraries is controled there is no problem providing the needed library for each one.
The problem of security holes or bugs remains when you install an old version: you get the features of that version.
Best software solves security problems or some bugs with the old version even when there is a new one, versions are maintaned till a given date after the software is deprecated.
I understand dt won’t be able of doing that as it requires more development, I can keep with DT 4.0 limitations for now.
and mostly an improved performance.