Accidently installed Catalina I did not have a time machine back up and want to go back to High Sierra 10.13 While installing a security update I accidentally installed Catalina. I do not have a Time Machine backup for my High Sierra 10.13. So now I have made a Time Machine backup in Catalina and my plan is to wipe the HD and re-install High Sierra. The thing is, is that some of my software will not work with Catalina and I really need them.

Now my question is, once delete everything from my Mac and reinstall High Sierra, what process can I use to bring back the files and apps since Time Machine won’t restore a backup from Catalina to High Sierra?
Manually copy the files over from Time Machine.

Just because you can’t restore doesn’t mean you can’t browse and copy.  Understand, that you’re not going to restore applications and settings, but you’ll be able to bring over your data with no problem.

In fact, I’m doing this exact process with my new install of Catalina rather doing a restoration or using Migration Assistant.