As of September 2020, Azure AD does not support the HTTP-POST binding for SAML logout requests.
AD FS supported both HTTP-Redirect and HTTP-POST, so this means that not all apps on AD FS will be able to migrate to Azure AD successfully.
If your relying party only supports the POST binding, Azure AD will ignore this, and send the LogoutRequest via a GET request anyway (if a logout URL is specified), so this will likely cause an error in the relying party if they are not able to process the GET parameters.
See suggested feature, which I recommend you upvote if this is functionality you need:
https://feedback.azure.com/forums/169401-azure-active-directory/suggestions/41283820-support-post-binding-for-saml-logout
Azure AD and SAML single logout POST binding
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