Hope you're doing well.
If you are searching for a specific user by name, which enables you to evaluate the lastSignInDateTime, you can use-
If you are looking for list of users with a lastSignInDateTime before a specified date, you can use-
https://graph.microsoft.com/beta/users?filter=signInActivity/lastSignInDateTime le 2019-06-01T00:00:00Z
Please note that APIs under the /beta version in Microsoft Graph are subject to change. Use of these APIs in production applications is not supported. To determine whether an API is available in v1.0, use the Version selector.
Hope this helps.
If the answer is helpful, please click Accept Answer and kindly upvote. If you have any further questions about this answer, please click Comment.