It never was FIPS-approved and likely will never be. The wireguard protocol used by Tailscale uses ChaCha20 for encryption which is not FIPS approved.
IPsec or TLS-based overlays which use AES encryption and NIST-approved ECC curves or (gasp) RSA for key exchange and authentication. They generally suck in comparison with wireguard, which is a clean-sheet modern cryptographic protocol.