Honestly, I've aside from React and Java (8 -> 21 is big but still not that big), there's very little software that I updated and noticed a major step change difference in the system. Once it works, its fine
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You can check this article about Uber migrating its Mysql from v5 to v8 posted here 3 days ago [1]. Among other things, they observed a "~94% reduction in overall database lock time." The before/after graph is pretty impressive. It also gave them window functions and better JSON support, which are two very big features.
[1] https://www.uber.com/en-JO/blog/upgrading-ubers-mysql-fleet