[Lombra]

Yes, they are safe. Don't worry. Tainting refers to addon code affecting the default UI code, which may break things in your UI. For example if you try to do stuff with your unit frames you may end up tainting them so that you can't click them to target. This only concerns protected actions, which essentially means spell casting and targeting. Tainting may seem rather complex as it can spread a long way to an area you thought completely unrelated. Sometimes it is easy to avoid, but for now it's probably easier if you don't worry about it until you notice that something isn't working.

So to "edit" the default UI you have a few choices. You're not editing as in replacing the Blizzard_Collections.lua file, instead you need to redefine the code in your addon. In this case you would probably want to replace the function that you found that code in.

So make a new CollectionsJournal_UpdateSelectedTab function in your addon, copy the original body, and edit as you like. You might need to do it differently ultimately, but you can try this out for starters.