I just tested on nightly. I feel a confusion, this is a good catch IMO:
If you don't require any tag (eg. 0 to 2 or even 0 to 0 for both primary & secondary), the user can post without any tag but will see the tag prompt and will have to confirm without having selected anything.
I think there should be three scenarios here:
tag requirements > 0 : always show the tag prompt
tag requirements >= 0 : show a less invasive prompt (like: "do you want to add tags to your discussion?" etc. with buttons according to the choices)
tag requirements == 0 : no prompt at all
As of your use case @orschiro it would still be insufficient: in case of a tag requirements >= 0 (0 for users and >0 for admins), you couldn't get rid of a minimal prompt without an extension which would for example create a general permission to use tags.
A workaround would be to set a tag requirements == 0 ("0 to 0" for both tag types) and, as an admin, to always check "bypass tag requirements" when editing the posts. You could do this right now, but there is still a little issue to fix (the tag prompt appears even when 0 to 0 tag possible)
edit: I just opened flarum/framework3444