In most cases you can use the HTML / Rich Text toolbar for design when creating an Email.
It is therefore possible to use osTicket variables.
In the Filter Action Section under the Send Email action: You cannot use HTML / Rich Text and therefore no variables. The question is how can variables be used in this action
Using HTML/Rich Text in filter action section - send Email
You can still use them. You will just need to either copy/paste them from templates or remember the ones you want. They are pretty simple to remember once you know the structure. You will also need to apply this pull request:
With this being said, we hope to extend the Variable Typeahead feature to the Filter Action of Send Email in the future. I would encourage you to go to our Github page and search for a feature request in the Issues tab to see if this has already been requested. If so, add an update to the thread saying you would like to see this added. If there is not already a feature request for this, add one so it gets some traction.
Cheers.