Perhaps one of the biggest uses of 9p, globally, was google's gvisor, which runs an unimaginably large (to me anyway :-) amount of compute in Google Cloud. gvisor:
I suspect more bytes moved through 9p on gvisor in a day than moved on all other platforms since 2000. All I can say is, it's a lot.
gvisor used to use 9p (my fault :-) and, although we started with Andrey and Lucho's go9p, at some point it was replaced by Chris Koch's implementation. It's quite good.
I'd argue that this may be the most real-world-tested Tflush handler you'll see. I have seen Tflush handlers that just return, having done nothing, and it's possible that in many cases, that's good enough. But Chris's code is VERY heavily tested with real workloads.
I also know, as I saw it many times, that the Plan 9 kernel Tflush could at times get extremely confused. When we ported it to Akaros, we even saw cases where Tflush would run out of control and exhaust the XID space, sending flush after flush as fast as it could create them.
HTH
ron
p.s. FYI, when we first started gvisor, we had it in mind that it could support Plan 9 binaries, and planned accordingly, but that was never tested.