License - How to cope with users on longer leave (maternity, sabbatical etc)

If you plan to continue using those accounts at some point, best leave them licensed. You can switch them to a "lower" plan, such as E1, depending on the features you have enabled.

Technically, you can put a hold on the account, then remove the license without worrying data will be lost. But that's a violation of the license agreement, which states that you require licensed users for holds. The other option is to use the Inactive mailboxes functionality, but that requires removing the user object, which makes the "recover" operation more complicated. Lastly, some organizations convert the mailbox to a shared one, which doesnt require licenses (again, depending on the features you have - if you put it on hold, you still need a license).