when did this “chumra” begin?
I visited Israel years ago and yes, I rode the #1 bus and the #2 bus through Geulah and Meah Shearim. Chareidi men & women generally arranged themselves so that they did not sit next to each other, but no one made a fuss about who sat where.
Health:
I’ve been on the B11 bus in Brooklyn which runs through Boro Park & is utilized by many frum men and women & I have never seen anyone insist that the seating had to be arranged to keep women and men apart.
What’s next? I’m not allowed to eat food on the bus if it doesn’t meet your particular standards for kashrus?
Your personal chumrah is YOUR issue. No one else is required to sacrifice for it.