When Apple announces macOS Catalina they were clear which Macs would and would not be supported, however, this doesn’t mean that older devices wouldn’t be able to run them.
I have an old 2009 Mac mini which I use as a plex media server which was not capable of running macOS Sierra yet according to the site it would be able to run Catalina unofficially.
I decided to give the install a try and the installation took a couple of hours whilst I freed up some external storage space. The guide was spot on and easy to follow and I have been successfully running Catalina for around 4 days now.
The performance is a little slow which I expect with such an old machine but that’s about the only limitations that I have encountered.
If you are looking to breath life into your old machine take a look at Dosdude’s website.