When publishing a layer for web, the following error is received if any in-memory joins are present:
It would be very nice if there were a tool for converting such join to a permanent one, rather than requiring the user to unjoin/rejoin with the Join Field tool. It would be even better if that tool were accessible from the Messages pane of the Share As Web Layer pane so you can take care of the issue right then and there in one click.
The tool itself would be useful in circumstances that don't involve publishing to web, too.