One thing that has changed, gradually over the past 30 years really, and then more suddenly in the past 15 is how leisure centres are funded.
The old model where I live was heavy subsidy and the sale of cheap 'casual swimming' tickets. So it was easier to say the pool is closed to men right now, tickets for women only. People expected to turn up and swim at the sessions available to them.
The model has now moved to less (or ideally very little to no) council subsidy, so the bread and butter income now comes from memberships (usually combined swim/gym although you can also get a swim only at mine). Members tend to expect access when it suits them in return for their membership, not awkward timings and restrictions.
This puts women only sessions in a difficult position- put them at peak times before or after work, you might put off potential members or have members leave, jeopardising centre income- margins are often thin anyway. Put them at non-peak times, noon on a Wednesday, or 9pm on a Thursday and they might be poorly attended and appear unwanted.
Even harder when you account for the fact that the early evening will often be kids swimming lessons, already eating into peak time. Swimming lessons are reliably lucrative though.
The exception is where large numbers of users come from religions that demand segregation for modesty's sake. Member demand is likely to be sufficient to make separate male and female sessions viable. Whether that is a good thing or whether it brings up its own questions (if you're a woman at a mixed session are you immodest?) probably depends on your pov.
Maybe a very cold observation, but it would be my view on what has changed.