Hopefully having several offline installs of various versions in parallel doesn't cause problems.
Qt being under LGPL, there is nothing that prevents anyone else from redistributing open-source builds of Qt - Debian / Ubuntu through apt, MSYS2 (pacman -S mingw-w64-qt5-whatever), homebrew (brew install qt), conan (conan install qt/5.14.0@bincrafters/stable), vcpkg (vcpkg install qt5)...
On Linux/BSD though, obviously most people use the in-built package managers where the Qt version is recent enough for their purposes.
Have you tried it? Does it ask after the download?
Oh it looks like you obtained that launcher from the old "archives" directory?
http://download.qt.io/archive/online_installers/2.0/
It's unfortunate that only has an old Linux installer. Saw it yesterday, but needed an OSX downloader and was out of luck. :(
http://download.qt.io/official_releases/qt/
At least, after disconnecting the internet connection then ignoring the subsequent error (ironic for an "offline" installer!), it doesn't require a Qt account.
For now we (sqlitebrowser.org) will keep with the easy approach of using the official Qt installers.
If Qt does something even further dumb though, we'll likely move to compiling Qt ourselves. And probably then make installers (.msi/.dmg) for anyone else that wants it too. ;)
Grab the official Qt installer, and try downloading anything:
http://download.qt.io/official_releases/online_installers/
It will not work unless you sign in with a Qt Account.