Although installing PVE using the PVE ISO is usually preferred. Using a non-preferred installation method just to avoid having to spend a few minutes clicking 'Next' in the PVE installer is probably not a great idea.
I think the pricing page at https://proxmox.com/en/proxmox-ve/pricing answers your question pretty well:
In other words: yes, not having a subscription for testing is fine, because you don't need reliable software updates and security enhancements in that case. You may need technical help and...
PVE Admin Guide, chapter 5.9
Note that systems like Ansible don't remove keys if they don't explicitly have the 'absent' state. Also, you can add a file to OpenSSH's 'AuthorizedKeysFile' and do key management as usual there.
I understand, and while that is technically true, I think the message is confusing from a practical standpoint :). See here the first question about this message: https://forum.proxmox.com/threads/first-backup-of-pve-vm.82871/#post-364565