The most notable new feature is the rewritten command substitution parsing code, which calls the bison parser recursively. This replaces the ad-hoc parsing used in previous versions, and allows better syntax checking and catches syntax errors much earlier. The shell attempts to do a much better job of parsing and expanding array subscripts only once; this has visible effects in the `unset' builtin, word expansions, conditional commands, and other builtins that can assign variable values as a side effect. The `unset' builtin allows a subscript of `@' or `*' to unset a key with that value for associative arrays instead of unsetting the entire array (which you can still do with `unset arrayname'). There is a new shell option, `patsub_replacement'. When enabled, a `&' in the replacement string of the pattern substitution expansion is replaced by the portion of the string that matched the pattern. Backslash will escape the `&' and insert a literal `&'. This option is enabled by default. Bash suppresses forking in several additional cases, including most uses of $(<file).

That's literally what the link gives! :)

Well they asked for a TL;DR. The link is three pages of dense text and much of it isn't major changes. Seems reasonable for to ask for a summary of the big changes.

Isn't it what the link provides?