I use this package when testing third party libraries on Travis. See https://github.com/symfony/psr-http-message-bridge/blob/master/.travis.yml#L41

Could/Should I really use flex for that?

Yes, see symfony/flex#409 and symfony/symfony#28199

My case is exactly same as @Nyholm .

from DX perspective, it's easier to set up force/lowest package as dependency and all done, vs require flex, configure it and set SYMFONY_ENDPOINT env var :(

Great, so you would just need to composer global require --no-scripts symfony/flex in your CI and export SYMFONY_REQUIRE=~4.1.0 instead. I don't see any DX issue here. force-lowest and lts packages are far from being trivial to use actually, they're much more difficult to work with correctly.
And the maintaince burden doesn't justify keeping them.