Problem/Motivation
Comments and nodes both offer 'preview' functionality: when you're creating or editing the entity, there's a preview button on the form, and it will show you an approximation of what your node or comment looks like.
However it's not a real preview, because it doesn't and can't show the entity in its usual context (different view modes, or the comment in the hierarchy) - it's just a special one-off, very hacky (relies on $node->in_preview being set dynamically) render of the entity.
Steps to reproduce
Proposed resolution
For nodes, we already have a full, in-context preview functionality via the workspaces module. You can even make multiple related changes to different entities and preview how they will look altogether. Great, why keep the 15 year old hack around when we have that?
For comments, I do not remember being given the opportunity to preview a comment in any non-Drupal website for many, many years. Formatting is already 'previewable' via wysiwyg, this is enough.
Remaining tasks
Decide that we want to get rid of this feature. Then remove it entirely with no replacement.
User interface changes
API changes
Code responding to ::in_preview won't have to do so any more, but otherwise it will just be a button removed from the entity forms.
Data model changes
Release notes snippet
