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I know I know, another "help me choose a distro" thread
(lemmy.world)
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You've picked pretty stupid criteria, but if you're adamant on it, as another commenter said Slackware is probably one of the best options.
Fedora dances with Flatpak quite a bit, but you could double check if RHEL does (since that's what Fedora is based on).
Again, while Slackware (and possibly RHEL) fit your criteria, your criteria seems pretty silly, and you're likely to walk into bigger (and harder to solve) problems on more obscure platforms.