flat_map/flat_tree can be implicitly copied as I just realized while digging into an issue, e.g.

https://cs.chromium.org/chromium/src/components/safe_browsing/android/safe_browsing_api_handler_bridge.cc?rcl=edff7d305f82de81cec263413897a7d126661da7&l=191

  api_task_runner_->PostTask(
      FROM_HERE,
      base::BindOnce(&SafeBrowsingApiHandlerBridge::Core::StartURLCheck,
                     base::Unretained(core_.get()), callback, url,
                     threat_types));

most sets are small (in this case it's mostly IDs), I'm not sure it's super important but it still seems to me like unintentional copies are generally bad and can be specifically horrible depending on the set..?

I tried making the copy constructor explicit but it's a major pain : https://chromium-review.googlesource.com/c/chromium/src/+/901604

In particular GMOCK isn't happy, things like
  EXPECT_CALL(favicon_service_, SetFavicons(base::flat_set<GURL>{kPageURL},
                                            kManifestURL, kWebManifestIcon, _));
use the implicit copy constructor for the args...