DefaultPublicFolderMailbox is technically used for modern public folder access in Exchange Online. It can be set per user mailbox to tell Autodiscover which SMTP address to return in the PublicFolderInformation section of the XML response. EffectivePublicFolderMailbox is a property that cannot be set, but it is populated with one of the remote public folder proxy mailboxes that is set in the Organization Config, and as stated previously, it is completely random. The Microsoft article mentions nothing about this, but I found that you can force Exchange Online to return a mailbox of your choosing by setting the DefaultPublicFolderMailbox attribute. Changing the DefaultPublicFolderMailbox property changes the EffectivePublicFolderMailbox property as well.
And the command you shared above can be used to set the DefaultPublicFolderMailbox attribute for a mailbox
Set-Mailbox -Identity test123 -DefaultPublicFolderMailbox PFMailbox1
And according to the introduction of the parameter: -IsExcludedFromServingHierarchy, However, if an administrator has set the DefaultPublicFolderMailbox property on a user mailbox to a specific public folder mailbox, the user will still access the specified public folder mailbox even if the IsExcludedFromServingHierarchy parameter is set for that public folder mailbox.
And If you want to update the command for all users, you could use the command below to meet this need:
Get-Mailbox | Set-Mailbox -DefaultPublicFolderMailbox PFMailbox1
Below links for your reference as well:
Legacy Public Folder Coexistence in Multi-Site Hybrid Deployments
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