Ok, I should add I did a bad thing and didn’t do a time machine backup before I installed Catalina, but I installed it at the 9th beta so felt it was an ok risk. But when you say partition, I didn’t necessarily install Catalina on a separate partition. Or perhaps I did and didn’t realize, but I didn’t explicitly do it.
No, I meant, when you install OSX (for like the past 8 years or so), it makes a hidden "recovery partition," which is made up pretty much of the contents of the "installer" app...
I don't recommend installing Catalina on a partition separate from Mojave or whatever else you have, unless you REALLY know what you are doing, and understand the implications of what is happening in the Catalina install. (the whole separation of user/system partitions...
With no backup, you will likely be OK, unless you require 32 bit apps....