If your checklists are set with Direct manager approval, not sure you can get around without updating the user’s manager relationship to one that is not on leave. Haven't tried this but you could create a new manager type and install and “indirect manager” - not sure if this would work., It might be scalable but that depends on how you manage your users (manually, or using som automation to keep the profiles up to date.

check out this KB article

https://help.docebo.com/hc/en-us/articles/360020082000-Setting-up-My-team-as-a-Superadmin#subtitle-2

Thanks ​@lrnlab! We currently populate an indirect manager field through our Workday integration, which allows us to target those managers when setting up checklists. However, we are looking for a way to remove the approval task from our PUs' when the direct manager is the recipient and on LOA. Ideally, it would be beneficial for managers to have the ability to assign a "delegate approver" in these instances. I plan to post an idea but wanted to make sure I wasn’t overlooking something first.

Please add th link to your Idea post so we can vote it up. Thx