I think texting has taken over writing, and emojis have taken over conversations sometimes! E-cards have replaced real letters and cards and thank you notes. We're buried in our phones and not engaging personally - so it takes away the facial expressions, the conversation, and the need to be present in the moment. So I think a lot of people have gotten away from the manners thing.
Miss Manners would say a few things. She would say that thank you notes are not out of style, RSVPs are essential, and people shouldn't have to chase others for an answer to an invitation. She'd probably say that your neighbor should have called you to discuss, or that you might have called her back to get her feedback on a few things to save yourself time.
My son had to either write a thank you note (before using the toy or cashing the check) or send a note back with the gift saying why he didn't want it. Obviously, he never chose the 2nd option. Either way, he was writing a letter. When he was 2, I wrote it and he scrawled across it with a crayon. At 3, he added stickers. At 4, he wrote at least the first letter of his name. At 5, he "dictated" the letter and signed his name. And son on.
In your case, I would probably call the neighbor and say, "Hey there, I got your message and sent you a list of suggestions. Did you get it? If not, I will re-send it. If so, was there anything helpful on it or do you need more?" That ought to prompt her to respond with a thank you (not a long letter - just a quick message of "Thanks - you're awesome!"), and if she doesn't, I'd just take my time helping out the next time. I'd still help her the next time, but I'd keep an eye on the gratitude thing just to decide how much energy to keep investing over time.
Your hubby is wrong. Yes, it was your choice to respond, but that doesn't mean she has no obligation to thank you. Ask your husband if it's his choice to give you a birthday gift and you have no obligation to acknowledge it. If it's his choice to do X chore or give the kids allowance, then it follows by his logic that absolutely no one ever has to say thank you to him. Try it for a day, just not saying thank you to him for ANYTHING, and see what he says.
And just take a look at the endless Mamapedia questions (and issues in our general lives) of people planning parties, kids' birthdays, and weddings, with a huge percentage of people not RSVPing. Hosts are texting and emailing parents to find out if their kids are coming or not, they're buying extra food for those who might "just show up", and getting frustrated with wedding catering and seating arrangements (hugely expensive) just because people are a) rude or b) waiting to see if they have a better offer for that same time period.
I'd rather have a friend like you!