Why is the 3.14 version required ?

Note that increasing the minimum cmake version is a non-trivial operation,
because it implies dropping support for some platforms which do not offer the newly enforced version.

In this case, dropping support for 3.10 means dropping Ubuntu 18 bionic, which is notable.
Other examples are: dropping Fedora 28 & 29, OpenSuse 15 or Alpine LInux 3.8 & 3.9 for example.

Ubuntu 18 bionic is still within its extended support range, so it's not great,
but it can be compensated by bringing something of value in exchange,
in this case "iOS-derived compilations".

Now, assuming we nonetheless want to increase the minimum version,
the rest of the cmake script may have to be adjusted due to the change of version.

In this case, the impact looks limited:
mostly, we have a ZSTD_CMAKE_POLICY_VERSION set to 3.13,
which feels somewhat inconsistent with a minimum version set to 3.14.
It probably needs to be updated.

Note that, in contrast with the minimum version,
the policy version can be updated to some much newer version,
as long as a test proves that the script works fine under the tested version.

@Cyan4973, thank you for the clarification.
Framework builds were added in CMake v3.14 and I thought that it's probably sufficiently old (Oct 2019)...
I can revert it back to v3.10 and the framework properties that I added in this PR would work just fine with iOS-Cmake third party toolchain and framework builds would be possible with slight changes in the configuration command:

cmake -B build -G Xcode -DCMAKE_TOOLCHAIN_FILE=<path to ios.toolchain.cmake> -DPLATFORM=OS64 -DZSTD_FRAMEWORK=ON

Generally, our support policy is closer to 10 years,
thought there can be exceptions at times.

I like the second solution you propose.
It probably deserves to be documented,
typically within https://github.com/facebook/zstd/blob/dev/build/cmake/README.md .

All done!

Would it be possible to have a test in CI that checks this functionality ?
Otherwise, the risk is that this functionality might be broken by some future update, and we wouldn't notice it.

Would it be possible to have a test in CI that checks this functionality ? Otherwise, the risk is that this functionality might be broken by some future update, and we wouldn't notice it.

It's possible to create a build job with ZSTD_FRAMEWORK flag, which cannot run the tests, but it can confirm it builds and validate the install.
I'll save this for another PR, if agreed.

it can confirm it builds and validate the install.

Yes, that will be good enough