Enable litigation hold on an Exchange Online Plan (Mailbox Plan)

There's a very limited set of parameters we can use for mailbox plans, so for this scenario you can simply run a scheduled PS script. Here's a sample article: https://www.michev.info/Blog/Post/1220/faster-script-to-put-mailboxes-on-hold-in-exchange-online

Or you can do org-wide hold as suggested above, though I find those tad more annoying management-wise.