Thanks for this recent article about Dropbox:
https://support.rstudio.com/hc/en-us/articles/200534477Using Dropbox with RStudio Projects
However, the advice about using Selective Sync will remove the .Rprojuser and .git folders from your computer (Dropbox will keep its cloud copy). So I hesitate to do this because this seems like it would destroy the project/git. It is only possible to exclude/ignore individual files with this hack: https://superuser.com/questions/469776/how-to-exclude-files-not-folders-from-dropbox-sync but not whole folders.
Maybe I need to make a copy, do Selective Sync, and then re-add? Or similarly, do Selective Sync, and manually download back to the folder? If this is what RStudio meant in its support file (above link), this extra step should be clarified.
Also, advice for Box Sync and Google Drive etc. would be helpful. It appears "Unsync" in Box Sync will also delete the computer copy and keep the cloud copy. So this won't work either. (unless, perhaps, I manually download and re-insert the .Rprojuser and .git folders)
Conclusion: It does not appear that Dropbox or Box Sync have "ignore" functions and RStudio projects/git and Dropbox/Box Sync don't play well together
UPDATE: the manual re-adding works for Dropbox. I Selectively [Un]Sync'd, then manually re-added the .git and .Rprojuser folders. The local copy of .git and .RProjuser are now free to be modified by RStudio without Dropbox's interference.
So my only remaining request is for RStudio to clarify this extra step involved after unsyncing in the support page: https://support.rstudio.com/hc/en-us/articles/200534477Using%20Dropbox%20with%20RStudio%20Projects