The AUR version like php84-iconv just brings too many things, including the php itself.
It would be nice to just have a php-iconv because Manjaro version of php is already compiled with iconv, all we need is php-iconv just for interface function wrappers.
I’ve moved your thread to this category as it’s not a support issue and it seems best located here (it might get moved again though.) 
I guess you couldn’t start a new thread here; if you can’t reply to this let us know.
Since the normal php packages includes php-iconv , why is an extra split package needed? Just install php.
And even if a split package would exists, it would depend on php. Which means the php package would be installed if you try to install the iconv split package.
Many things might be nice.
Unfortunately, a request like this that amounts to little more than a personal whim; with no significant benefit to the wider community; is unlikely to be given serious consideration;
@xabbu has summarised it succinctly in their post (above).
Consider instead using the standalone bsdconv which is available from the AUR – it’s quite old, but still works as expected (much like me, really);
pamac build bsdconv-git
Once installed, type bsdconv for basic usage.
Regards.