MIGRATION TOOL: Is it safe to erase iPhone without backing it up?

Hello there. Since iOS 12.4 came out recently no one has been able to answer my question. I just transferred all my data from my old iPhone to a new one using the Migration tool. The thing is, I didn’t back up my old iPhone before doing so and I am wondering if it is safe to ‘Erase All Content and Settings’ from the source phone without affecting the new one. I really don’t want to lose any data.

Is it necessary to back it up via iCloud/iTunes to keep it even if the Migration tool didn’t require it itself? And if so, should I back up the new or the source iPhone? Watching tutorials I noticed people get this message after the transfer is complete [picture attached] but I didn’t. I hope someone can help me out.