I believe tags are published for all plugins, which you can use in your pipeline. For example, you will notice multiple versioned tags (1.11, 1.2, etc) for the slack plugin: https://hub.docker.com/r/plugins/slack/tags
If there is a specific plugin that does not have version number tags, I would encourage you to find the specific GitHub repository and work with the maintainers to create a tag.
Yes. I saw some “old” tags on repo. but I think there’s no tag support for latest images.
For example on Drone docker repo, latest image uploaded 13 days ago, but there’s no static tagged one for that. Last tagged one is 17.12. Which uploaded a month ago.
It will be nice to have image always with static tags. not having latest only.