@misemoi
I've no idea what field you're in but even in the public sector it depends on your grade, your organisation, and what part of the organisation you work in. Often the individual 1st & 2nd line manager as well. That said, in 8 years, two Departments, and 6 managers (I know....) I've had no issue getting 4 weeks shorter working year approved and I don't expect any problems getting it in future.
In my previous role & department on the other hand it was absolutely unavailable, as was a reduced working week. Even parental leave was only available within the strict legal entitlement ie max 2 periods of minimum 6 weeks at a time. This was justified by management as a requirement because it was a frontline role with shift-work. (it was and remains very much a revolving door work location, whereas most departments people move around internally rather than moving out).
But depending on your definition of "junior" it's very much the case that the higher you go the less available flexibility is available within the Civil Service at least. I'm a middle-manager and am lucky to have so far been facilitated as much as I have been; if I went 1 step up the ladder I'd be working full time until I retire, with absolutely no hope of getting any kind of leave to which I wasn't legally entitled .
But in general, it's getting back to the time Vs money tradeoff. Money in the bank/pension/investment fund is nice and all, but I'll never ever get back the time I don't spend living, and that's doubly or triply true as a parent in my view. Sacrificing a portion of my salary and limiting my career is a price I'm more than happy to pay, but many others are clearly not.