InDesign

If you do decide to delete earlier versions make sure that you do so using the Uninstaller that you will find by clicking on the 3 horizontal dots to the far right of the version to be deleted in the apps section of the Creative Cloud Desktop app. Never delete a Creative Cloud app by trashing it in the application folder.

As a rule I never delete a version until after I've worked with the new one for awhile and feel comfortable with it. Since files created with earlier versions will always open with the newer one the only real issues are, as Peter said, making sure that any third-party plug-ins are updated.

It's generally considered safe, and the I beleive the official line is you should do it, but there are some real-world caveats.

Third party plugins probably will need to be updated to run in a new version.

Text is likely to reflow if you open and edit a legacy file with long runs of text (or even some short ones), so you need to be vigilant for new oversets or other changes.

New versions have new bugs that can affect your particular workflow in ways that make it impossible to do what you need to do until they are fixed.

I think working with legacy files created with Creative Cloud versions may in theory be easier than Creative Suite versions (and here I'm talking about exchanging files with other users) because all CC subscribers should have access to current verions, as a pragmatic person I keep old versions around just for those rare times I need them.