macOS Catalina requirements

I attempted to back track and locate the Apple content that I had read that mentioned the storage minimum was 18 GB for new macOS installations, but could not find it again. Realistically, I have been advising 20 - 22 GB as the absolute minimum just for breathing room. I imagine with all the extra, and IMHO, unneeded features in Catalina, that it will require substantially more room than Mojave.

Often this is caused by time machine local snapshots. You can thin them: in Terminal, paste the following line

sudo tmutil thinlocalsnapshots / 10000000000 4

type your password when asked (nothing appears when you type it) and press enter.

No one is going to know until Catalina is available. As an aside, you need to keep sufficient empty disk space in order for the OS to function properly - an absolute minimum of 10 - 15 GB at all times is reommended.

I agree with that - the new features will surely increase its size. Personally, I have always maintained a minimum of 200 GB since I do a fair amount of editing/rendering video which does require plenty of empty space, but for "general" recommendations, I've been going with the majority here which I will now have to adjust. I also seem to remember having seen something about 18 GB, but cannot remember where that was.

I find no reason to expect Catalina to take significantly more space that Mojave.

I expect them to take more or less the same.

I totally agree that having only 20GB free is a very bad idea - even if one is not upgrading the OS.

Apple has added a boat load of new functionality. We shall see what this does to the operating system size. One that has less than 20GB of remaining storage will soon encounter other issues that will ruin their day if they do not address the low storage issue.