It was first added in SSLeay 0.8.1 which predates OpenSSL let alone the LibreSSL fork.
It probably doesn’t exist in BoringSSL but neither does a lot of things.
> On 28 Nov 2023, at 00:21, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> Michael Paquier <michael@paquier.xyz> writes:
>> Interesting. I have yet to look at that in details, but
>> BIO_get_app_data() exists down to 0.9.8, which is the oldest version
>> we need to support for stable branches. So that looks like a safe
>> bet.
>
> What about LibreSSL? In general, I'm not too pleased with just assuming
> that BIO_get_app_data exists. If we can do that, we can probably remove
> most of the OpenSSL function probes that configure.ac has today. Even
> if that's a good idea in HEAD, I doubt we want to do it all the way back.
>
> I'd be inclined to form the patch more along the lines of
> s/BIO_get_data/BIO_get_app_data/g, with a configure check for
> BIO_get_app_data and falling back to the existing direct use of
> bio->ptr if it's not there.
>
> regards, tom lane