This is important stuff. If nothing else, when (and preferably before) you make the leap to Catalina, read the “summary recommendations” about a page down. The first two:

• Before upgrading to Catalina, make one last backup (or two!), then turn Time Machine off.

• Once you have upgraded, turn Time Machine back on but using a new backup set which won’t then be accessible from macOS 10.14.x or earlier.

From Apple’s Time Machine support document, updated for Catalina:

If you create a backup on a Mac or partition with macOS Catalina, you can only use that backup on Mac computers or partitions with macOS Catalina.

Seems to me, this is a sea change for Time Machine.