I'm not going to read all the comments because IME they are usually full of annoying assumptions made by people with no direct experience.
I haven't been in the FB group mentioned - it's likely to be free of those comments, but I'd be cautious as IME hyper-specific parenting issue FB groups can end up with a weird competitiveness that is not always helpful.
I liked the book Breastfeeding Older Children by Ann Sinnott.
DS1 fed until he was 4y 3m, I didn't think he was close to weaning, but when it got near to the end, he would only latch for about 3 seconds and then stop, he was also only wanting it a couple of times a week.
I realised one day he was ill so I offered him BF and he said no, and then I thought oh, OK, he's weaned. He didn't ask again for 6 months and then suddenly did after seeing a friend BF her newborn. I didn't want to restart then so I said no, but did feel guilty about it and wondered if I should.
I will say I used to go to LLL meetings and even long after he'd stopped in the day (3ish) he used to climb onto my lap and want to be attached the whole meeting, seeing other children feeding. So I think it can be true that them seeing younger children feed can sort of remind them and make them want to do it. I have also had chance conversations with I think three other people who have mentioned to me that their older children continued to feed sometimes when there was a younger sibling, up to 5/6 years or so. I would not have known if I hadn't mentioned feeding until age 4 because people typically don't mention it. They weren't especially "hippy" types either.
For DS2 who was born 10 years later, I only fed him until age 2.5. I was pregnant and experiencing nursing aversion, as well as wanting to night wean before the baby came. I thought I'd feel guilty for cutting him off so much younger, but he was fine, I was fine and it was OK. I thought maybe he'd want to feed when DS3 arrived, but he didn't at all. DS3 is now 21mo and not ready to stop yet. DH is getting a bit antsy, but I kind of feel like I'm not going to push it unless there's a clear reason that it would be of benefit. That's how I'd have to frame it for me. I don't mind, and it's not hurting him, so in order for me to make a change, why would I unless there's a reason to? If it ain't broke don't fix it basically. I don't think it causes them any social problems, at school etc, because they just don't think anything of it to mention it to others. It's as normal to them as any other aspect of their bedtime routine. I read that children will naturally eventually self wean when they get their adult teeth, so no need to worry about them never stopping.