Indy statically links to OpenSSL on iOS. Static-link .a files for OpenSSL are available at https://indy.fulgan.com/SSL/.
Yes, per Apple's recommendation in the link above, you can do that on both iOS and OS X. Apple does suggest using a known version. I can't tell what version these are - the archive files in OpenSSLStaticLibs.7z are dated April 21, 2014. Maybe they don't get much love.
Static linking will add considerably to the size of the app's executable, depending on how many of the 5 architectures from the archive are linked in. Currently, 4 of the 5 architectures would probably be needed, 2 for the Simulator app (32 and 64 bit) and 2 for the iOS native device app (32 and 64 bit). Plus some bloat from Indy code.
If the app only needs to send an HTTP request or work with a SOAP server via WST, then that might be overkill. In contrast, using a Foundation-framework based client won't link in any additional code since everything the client code uses is already on the iOS device or Mac.