https://www.freshports.org/www/owncloud suggests that there is an owncloud package for PHP 8.0.
It suggests that there is such a package because that package exists.
I know it sounds silly, but that is indeed what is happening here.
And in fact it is possible to install one using "pkg install owncloud-php80 on FreeBSD".
Yes indeed - once again for the simple reason that such a package does indeed exist.
And this despite knowing that it is not executable?
Yep. Unfortunately that is the case in this particular scenario. However, this is not because the port maintainer is being lazy or did something incorrectly. The port was not actually intentionally modified to offer a package for an unsupported PHP version. Instead, the port just says "this needs PHP" and the underlying port infrastructure takes care of the rest. In case of PHP, this means automatically creating port flavors for the various
available PHP versions. Until PHP 7 was EOL'd, there existed an equivalent
www/owncloud-php74 package. But then PHP 7 got EOL'd and eventually removed from the ports three. When that happened, that flavor also disappeared.
So here we are: Having a port requiring PHP 7 while PHP 7 is rightfully not available in ports anymore.
The only party to blame here is upstream (i.e. OwnCloud). PHP 7 was actively supported until November 2021 and received security updates until November 2022. Any legit development team would have made their product compatible with PHP 8 by November 2021 and if everything else falls apart
at least by November 2022.
As PHP 7 became EOL'd by November 2022 the corresponding FreeBSD ports tree removed support for it.
As stupid as it sounds: Right now you have no way of getting OwnCloud to run on FreeBSD without jumping through a lot of hoops. And that is good that way. EOL'd software
must be removed from the ports three for security & maintenance reasons. Anything depending on an EOL'd package will just go down the drain.
Have a look at Chromium. It relied on Python 2 for months to years after it was EOL'd. Not so long ago there was a time where one simply couldn't just install Chromium on FreeBSD for that exact reason: The Python 2 dependency disappeared
Now, this might suck but it is what it is. Personally, I feel like the security, maintenance & ports team of FreeBSD is doing an excellent job in this regard.
What I can recommend you is having a look at NextCloud instead. It's an OwnCloud fork and works really well (despite it being written in PHP). I run several instances myself without any major issues.