downgrade to older OS

Many people buy Macs with minimal RAM in order to save money, but this may spell problems running newer systems which are RAM hungry. Then the people find they cannot upgrade RAM in their Macs anymore since Apple started soldering in RAM over the last decade.

To get to your question...

Downgrading isn't easy unless you have a clone or Time Machine backup. Do you have one of those?

Revert your Mac to a previous macOS version [with Time Machine] - https://support.apple.com/guide/mac-help/mh15216/mac

https://communities.apple.com/fr/thread/250788825?answerId=251498564722#251498564722

"It is possible to attempt to return to the old system before installation even without Time Machine backup. Mac Os sets up an internal backup system (Snapshots) on special occasions and the system update is one of them.

In short, it is necessary to start in recovery mode (cmd + r during the boot) then to request the restoration Time Machine. There select the system disk (Macintosh HD in general) as Time Machine source and see what is on offer. With a little luck will appear a date / time prior to the update with the correct system version. If so, select it and then start the restore." [Google Translation]

There is no macOS downgrade. Never has been.

Restore your backups created when you were using Catalina; revert.

Or wipe, and reload Catalina.

Files and documents with formats that were updated with Big Sur may or will not be readable with Catalina.

Or try loading macOS Big Sur on and running from an external SSD, if you’re running one of the old 5400 RPM HDDs in that 2014. Thunderbolt preferably, or the fastest USB you can find.

Upgrading memory in a 2014 is not feasible, so if you don’t already have 16 GB there’s nothing that can be done.

Or look at replacing the Mac mini late 2014 with a more capable model.

MrHoffman wrote:

There is no macOS downgrade. Never has been.

Restore your backups created when you were using Catalina; revert.

Or wipe, and reload Catalina.

Files and documents with formats that were updated with Big Sur may or will not be readable with Catalina.

Or try loading macOS Big Sur on and running from an external SSD, if you’re running one of the old 5400 RPM HDDs in that 2014. Thunderbolt preferably, or the fastest USB you can find.

Upgrading memory in a 2014 is not feasible, so if you don’t already have 16 GB there’s nothing that can be done.

Or look at replacing the Mac mini late 2014 with a more capable model.

Who said there’s no downgrading?

Check the Related thread below

https://discussions.apple.com/thread/252591978

Downgrade from macOS Big Sur to macOS Catalina or Earlier by:

1. Internet Recovery
2. Software Update

Downgrading From Internet Recovery

1. backup your Mac.
2. Restart your mac
3. Hold Option Command R until the Apple Logo appears
4. Do the Steps Here - Erase and reinstall macOS - Apple Support
5. In Disk Utility, Make sure you selected MacOS Extended(Journaled)
6. Do the Setup

Downgrading from Software Update

1. Open App Store
2. Search for macOS Catalina or earlier
3. Click Get
4. Do the same as you update it to Big Sur
5. Follow the OnScreen Instructions from the Installer.

Hope this helps!

I am well aware of how to revert; how to restore backups, and described that. That’s what you linked to here too; reverting.

Reverting or restoring is in no way similar to what a macOS update or upgrade provides.

I’ve reverted, too. With varying success.

The longer it’s been and the more apps that got updated, the more difficult reverting gets, whether due to user changes made since the upgrade, or file and database format changes made by newer app versions associated with the “failed” upgrade.

In this case, the nub here appears to be an underpowered-or-underconfigured-for-Big-Sur ~7 year old Mac mini. Which’d be fodder for upgrade, just given its age. Reverting—or staying on an older version—is absolutely reasonable here and for similar cases, though not a long-term solution as the network connectivity and app support will age out.