32-bit apps use a sub-set of the same instructions that still exist on current 64-bit systems. Running 64-bit alone does nothing to eliminate any flaws, real or imagined, from the 32-bit side of things.
As @jarfil@jarfil has stated, 32 bit repos are being de-listed because no one can be bothered to maintain them(on a professional, full-time basis), and that lack of code/functional review could allow flaws to slip through. Meanwhile, a lot of those same 32-bit repos continue to exist(as community-maintained versions - my preferrence anyways) and can be accessed by interested users from most distros. They aren't blocked, just de-listed and unsupported by those distro maintainers.