Most applications do not do anything useful with in-band language tags. They never had widespread adoption in the first place and have been deprecated since 2008, so this is unsurprising. If you're using them in your strings and those strings might end up displayed by any code you don't control, you'll probably want to strip out the language tags to avoid any potential problems or unexpected behaviors. Out-of-band metadata doesn't have this problem.
As I said though, if you're in full control and only need to be compatible with yourself, you can do whatever you want.
in 2008 uft-8 was only ~20% of all web pages! Again, that deprecation fact is not meaningful, a quick search shows that rfc for tagging is dated 1999, so that's just 10 years before deprecation, that's a tiny timeframe for such things, so I agree, it's not surprising there was no widespread use.
Out-of-band metadata has plenty of other problems besides the fact that it doesn't exist in a lot of cases
As I said though, if you're in full control and only need to be compatible with yourself, you can do whatever you want.