Support Article: Can I use Triple Triangle under multiple years of Creative Cloud simultaneously?

Yes, you can use Triple Triangle software under multiple versions of Creative Cloud on the same machine simultaneously.description

Your licenses are counted by machine, not by InDesign version, so you will not use up extra licenses in this scenario.description

The only issue to be aware of is that when documents are saved backwards (e.g. from CC 2019 to CC 2018), the slug will become static content. It will no longer update. Consequently, you usually want everyone who is working on a particular set of files to be using the same year of Creative Cloud.