A nice feature is the use of some kind of text parser such as https://www.w3.org/TR/activitystreams-vocabulary/#microsyntaxes , detecting "to" property in text, mention tag types, or simple tag properties.
Maybe the "to" property should be filled with some kind of confirmation.
Also we could create some new conventions, or reuse some found in mail, for example use re:http://note.example.org to automatically fill the inreplyto property
or directly allow the use of properties as of https://www.w3.org/TR/activitystreams-vocabulary/#properties in content to directly fill the property.
Needs work
1.0
Code
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
To workaround this block I bumped my expired feature request in a new one https://github.com/ckeditor/ckeditor5/issues/18370 , Please re vote there...
and also made a PR in github: https://github.com/ckeditor/ckeditor5/pull/18371
I made some tests locally and mentions looks fine ;)