Recoverable Items Folder Quota is Used up for a cloud Mailbox on Hold

1) Same as when it reaches the quota for "non-hold" mailboxes, you cannot delete items, you get a warning and you might even get issues with mail flow.

2) No one, you have to monitor it yourself. Unless you want to get surprised by an NDR or warning in PowerShell

3) As above.

Here are some articles with more details, including how to clean it up:

https://docs.microsoft.com/en-us/office365/securitycompliance/increase-the-recoverable-quota-for-mailboxes-on-hold

https://docs.microsoft.com/en-us/exchange/policy-and-compliance/recoverable-items-folder/clean-up-deleted-items?view=exchserver-2019