Installing an older version of macOS could allow you to reuse an Apple computer that has become too old for newer versions.
As you most likely know, Apple updates its computers every year with new features and security patches. Unfortunately, some iMac or MacBook models may no longer be compatible with new updates and thus become obsolete. This is what is more commonly called: planned obsolescence.
Few users know this, but their computer does not become unusable and it is not necessarily useful to proceed with the purchase of a new model. Indeed, it is quite possible to reinstall an old version of macOS to continue to run the computer. Older versions of macOS are also much lighter on disk space, like macOS Tiger, unlike Monterey or Big Sur. Finally, some software may no longer work under new versions of macOS, so it is necessary to return to an old version of the OS.
This is not visible to anyone, but Apple leaves the download to the last four versions of macOS possible from the AppStore. The entrance is hidden, but it can very easily be found thanks to these links:
If your Mac is too new, the link will be unusable. Go to this link with an older computer and therefore compatible with one of the versions above.
Apple also allows downloading even older versions. Here, it is strongly advised to check the latest version that your Mac can run and then back up it.
Then just download the DMG files above, reset your Mac and then install the OS.