Client Authentication with Proxy SSL

To be clear, there are generally two options for allowing mutual TLS to pass through the BIG-IP, with client/server SSL profiles applied:

• ProxySSL - as described above. But note, ProxySSL can only work with non-perfect-forward-secret handshakes. That means it only works with RSA handshakes, and never with ECC, DHE, ECDHE, ECDH, etc. And since RSA handshakes have been mostly deprecated by all modern browsers, this option isn't terribly useful unless you control the browsers and are willing to force significantly weaker encryption.
• Client Cert Constrained Delegation (C3D) - https://my.f5.com/manage/s/article/K14065425. This is the modern way to handle the requested scenario, and works with TLS1.3 and below with modern ciphers.