very doubtful that bbpress would add this – you’d need to register a trac ticket
I have created my own plugin:
What it does is install the “autosave” plugin as provided on the TinyMCE website. This is version 4.9.4 which at the time of writing was supported by WordPress 5.2.4.
This plugin by design will warn you if you navigate away from a topic without pressing submit.
What is also good is that it is compatible with “TinyMCE Advanced” so it will not reset any changes you have made to the toolbar using that plugin.