How to get token access into dropbox without it expiring (or copy .httr-oauth file)

sydneyfr Здравко is correct; Dropbox is now issuing short-lived access tokens (and optional refresh tokens) instead of long-lived access tokens. Short-lived access tokens expire after a short period of time, so you'd receive a 401 expired_access_token error when attempting to use a short-lived access token that has expired. You can find more information on this migration here.

Apps can still get long-term access by requesting "offline" access though, in which case the app receives a "refresh token" that can be used to retrieve new short-lived access tokens as needed, without further manual user intervention. You can find more information in the OAuth Guide and authorization documentation. There's a basic outline of processing this flow in this blog post which may serve as a useful example.

For reference, while the creation of new long-lived access tokens is now deprecated, we don't currently have a plan to disable existing long-lived access tokens. (If that changes, we will of course announce that ahead of time.) That being the case, you can continue using existing long-lived access token(s) without interruption, if you have any.

Dropbox can't offer support for RStudio/rdrop2/drop_auth in particular as those aren't made by Dropbox, so you may want to refer to the support resources and documentation for those for information on what functionality they support for this.