...
Netflix has contributed an implementation of BBR to FreeBSD. See the main/sys/netinet/tcp_stacks/bbr.c source file and the commented commit by Randall Stewart from September 2019.
Microsoft Windows
Recent Insider Builds for Windows 11 contain an experimental knob to enable BBR in TCP, according to a presentation at the ICCRG meeting at IETF 112.
BBR in QUIC
BBR has also been implemented for QUIC (including BBRv2, see above), and is in active use on Google's servers. Source code can be found in the congestion_control part of Google's QUICHE project. This is also used by Chromium, the open-source upstream project for Google Chrome. The main logic can be found in the tcp_sender.{cc,h} files.
...