M.R.
I remember the earlier post well.
Please let this go. This is going to happen again and again for both your girls: Other kids will be their best best buddies then drift away. Or your own girls will be the ones who drift away. It is normal and natural. You are the one mourning the loss of what seems to you to be the first sweet little friendship your older girl had; but she, herself, is not mourning it as you are, so you need to let it go; she has. She isn't asking and asking for the other girl to come play, or talking about the other girl...at all, really, right?
It's great that you are having playdates and engaging her more with girls her age. The gap in maturity and interests between 5 and a half and seven is larger than you might think, and this young neighbor might well have been absolutely fascinated with your daughter a short while back and absolutely fascinated with someone else no. It is typical and not a reflection on your child or a case of the younger neighbor being mean; she's being five, and at five kids get very intense about something then can drop it suddenly. And I think you suspect that the parents might have somehow pushed her away from your daughter...? Even if they did, it might merely come from their directing her toward playmates who are either closer to her own age (which you too are doing with your own child) or directing her toward playmates who have interests closer to hers than maybe your child does right now. Or neither, and the girl simply stopped asking to see your child because she moved on to play with others.
Please find a way to shake this because if you can't, you as a mom will be hurt for your kids over and over as they grow up. Yes, we all get hurt for our kids, but this is such a typical and normal separation that it really doesn't merit this amount of mental and emotional space that you're giving it. You've grieved this, now try to focus on the new friendships.
If you get this heartbroken each time this bittersweet but normal thing happens, you will be truly crushed if she has real issues later with friends, like bullying or "frenemies." Let's hope those things won't happen, but if they ever do, she needs you to be resilient.