Drupal 8.9 and above have been updated to jQuery 3.5.1.
jQuery versions before 3.5 incorrectly used XHTML parsing logic when parsing HTML strings in documents with the text/html content type, which all Drupal pages are by default. jQuery 3.5 fixes this, but it's a breaking change for JavaScript code that relied on the prior, incorrect, behavior.
If you had code that did something like this:
$("<div/><div/>")
and if your intention with that was to create two sibling DIV elements, you will need to change that to:
$("<div></div><div></div>")
Code that creates a single element with self-closing syntax (e.g., $("<div/>")) is unaffected by this change. Although that is technically incorrect HTML (HTML does not allow self-closing tags on non-void elements), browsers simply treat that as an unclosed open tag and create the intended single element anyway.
Code that uses self-closing syntax for elements where that's allowed by HTML (void elements, SVG, and MathML) are also unaffected by this change.
This change affects all jQuery functions that parse HTML: $(), $.parseHTML(), $().html(), $().append(), etc.
If you find contrib modules or 3rd party libraries that are broken by this change, please report that in this issue.
