When logging is enabled for cataloging (CataloguingLog) and authorities (AuthoritiesLog), no details are shown for the deleted record or authority. The logs record a delete (with no details) and a modify entry for the merged item (with details).

When merging records and authorities it would be good to have some of these available (depending on how complex they are to do):

i)   show for the log action that the delete and modify actions were as a result of a merge, not a standard deletion or modification, for example: Delete (merge), Modify (merge)

ii)  record the details in the log of the deleted item (just before it was deleted) so it could be restored or recreated (for example: if it was accidentally merged)

iii) have a compare function (see bugs listed below)

iv)  have an undo merge function

See this Twitter thread for more details: https://twitter.com/elafrula/status/1457650198939897856

Other bugs I could see which have some elements of this:
* Bug 10417 - Add table to keep deleted authorities
* Bug 26207 - Compare values of system preference log entries
* Bug 26736 - Compare values of reports log entries
* Bug 14233 - Add logging support to notices and slips management
* Bug 28714 - Bib record change tracking action log