At the time of writing there is no version of Imagick compatible with PHP 8. Fortuntely, the version on the master brach of the Imagick repo seems to work.

Here's how you can use it.

1. Clone the repo somewhere on disk
2. Follow the steps mentioned here to create the extension.

cd <directory of cloned repo>
phpize
./configure
make

3. After those steps you'll find the extension in <directory of cloned repo>/modules/imagick.so. We need to move it to another directory to actually use it. First we got to find where PHP expects to find extensions. You can do that with this command:

php -i | grep extension_dir

For me that command returned /usr/local/lib/php/pecl/20200930, so I moved imagick.so to that directory.

4. Finally we need to let PHP use the directory. In your php.ini (you can find the location by executing php --ini), you should add this line somewhere in the file (it doesn't really matter where, I put it as the first line)

extension="imagick.so"

And with that Imagick should be installed. You can verify this by running php -i | grep imagick. If it outputs imagick module => enabled everything should be ok.

If your work relies on Imagick, consider sponsoring the primary maintainer Danack.