I think an autosaving feature would be beneficial to avoid losing annotations in case of crashes, power outages and so on, if the user did not save the changes manually. I think it should be an optional feature to avoid unintentional annotations overwrites and that it would be a nice touch if the autosave interval could be customizable (e.g. in a 1-10 min range).
*** Bug 406347 has been marked as a duplicate of this bug. ***
I came to request the same feature, but with one more option in mind. - Option to enable temporary/swap file containing unsaved changes This way you can can enable full autosave like original request, or a temporary file that contains any unsaved changes, or both where the temporary file would be useful to avoid losing any work, but can be excluded from things such as backups or file sync services like nextcloud/dropbox/google drive.
I also would very much like this feature. I just lost one afternoon of annotations and it definitely makes me question if I will keep using Okular or not. In my opinion it doesn't make sense having annotations if the user is required to repeatedly save the document to not accidentally lose them.