The feature request in our backlog for the next release.
Since Plesk Onyx you will be able to install memcache as a Docker container and configure http(s) proxy rule for it to map to your web app.
Docker and proxy rules are available in Plesk Onyx Preview already, you can try it here https://talk.plesk.com/threads/plesk-onyx-preview-and-feedback.337172/
Please install Memcached without needing Docker!
Thank you for your input. We will consider the possibility of implementing this feature in upcoming releases.