There is a suggestion JRASERVER-59132 opened in Atlassian Jira which you can comment & watch. At the moment out of the box Jira doesn't support this and I am personally finding this a proper behaviour model as it doesn't create a confusion like mentioning inactive users or assigning anything to them.
Well it's not always for them to act on. sometimes its as metadata.
I have the same problem with a JSM project where we have them as watchers or metadata fields and it blocks you from editing the issue once they are inactive
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