IME, when a formerly close friend fades a bit, they are preparing for divorce.

OP, what amount of contact could you initiate without being resentful when it's not reciprocated? Every person would answer differently. If you know, for you, do that. Let's say for example, it's every 3 months. Put it on the calendar and forget it. Contact her to invite her to something and if she says no, you're done trying for another 3 months. Probably after a year of this, I would have the last message be, "get back to me if you'd like to get together"

It's in their court. And if you run into each other somewhere, a random encounter remember, them saying, "we'll have to get together sometime" counts for nothing. If they add, "call me", that's the most obnoxious.

Is she a social climber op?

Were your sons at an all boys private school by chance?

some parents just keep in touch while kids are in school and while they need a parent friend at school then drop you. It is weird.

That being said she is either just a user type OR going through other things and does not have time for friendships

I've had this happen multiple times and it's so painful. But usually the reason is that they've found some sort of new social outlet that is taking up all their energy. One dear friend joined a church, got assigned to a "small group" and within months those people were her best friends who she sees multiple times a week. She has no time for me anymore and we were as close as sisters.
Another friend had a late in life baby and got in with a "baby group" and she is completely enmeshed with those women now. She does make time for me but it's not like it used to be and it's clear they are her priority.

It is hard to know what to do. I think one of my good friends is distancing and I am not sure why. I don’t want to look needy but I also have hard time understanding why there is this sudden gap. I am not planning to ask directly for an explanation (after all nobody is entitled to one) but there is a lot of frustration and uncertainty

Let me say this to the question of "being offended" - I don't like the way that's phrased because someone might not want to be friends with you for reasons other than that you offended them. I'm picturing a situation with some people I'm currently distancing myself from and if they asked if they had offended me I would honestly say no. So then they'd be left scratching their heads trying to figure out why I didn't want to be friends with them anymore.

I would instead say that things have changed and you want to understand why and you want to know if it is something you said or did. They may lie because they don't want to discuss it/have already decided that there is no coming back from whatever happened, but all you can do is ask. Just don't ask if you offended them.

I don't like guessing how people feel so I'd either let the relationship fade or I'd try to figure out what was going on. I wouldn't ask if I had offended her, I would just say honestly that you miss her and you'd really like to schedule a time to get together and then try to make it happen. Once you're in person you could be open and vulnerable and tell her how you feel and ask if you're missing something. If she still doesn't tell you or continues to act weird, then that sucks, but it doesn't seem like there's anything you can do about it.

Formerly.