This might be a problem with live preview only (Preferences → Editing → Show previews without reloading the page).
Before rMW7e9cac27d115: LivePreview: Show parse warnings and preview header/warning, it used to not show that message at all, unlike normal preview.
[[File:Start hand.svg|65px|left|预览|link=]] <div style="padding-left:80px"> <b><span style="font-size:115%;color:green">请注意</span></b>，这只是'''预览'''！内容还未保存。 <hr /> *您可以在编辑或修正内容后再次预览。 *若您对当前的修改感觉满意，请点击'''提交编辑'''按钮以發佈文章。 *若是测试，请改用[[WP:SB|沙盒]]。 </div>
I'll repeat that I don't think this is a train blocker. We've had very similar code in production for years and it's not bringing the site down. It's a minor performance issue at worst, and I think my proposed fix can be reviewed whenever someone has the time. I shouldn't even have worked on this today.
If you do want to revert regardless, then it's fine, but please also revert https://gerrit.wikimedia.org/r/c/mediawiki/core/+/597262 to avoid regressing this issue (and reopen the issues associated with that).