Looks like they’ve since updated the copy. Now it reads:
If you purchase Goodnotes 6 using the one-time payment, you can use Goodnotes 6 only on iOS/iPadOS/macOS, but for as long as Goodnotes supports Goodnotes 6 without needing to pay an additional fee, except in cases where new optional purchasable items are added.
Loosing family sharing is deal breaker
+1 on good value upgrade after so many years with GN5.
spent the morning creating new Disney stickers for my notebooks… 
So if I had been updating apps and failed to notice that goodnotes was changing me to six, then my GoodNotes five would disappear. Is that correct?
On their support page they say this:
I have already purchased GoodNotes 5, if I upgrade to Goodnotes 6 will I lose my access to Goodnotes 5?
No, you will have the option to revert to GoodNotes 5 from within Goodnotes 6. Please note, if you are on a free trial or have purchased a subscription to Goodnotes 6, toggling back to Goodnotes 5 will not cancel your subscription. Please follow the ordinary procedure to cancel your subscription or else you may be charged.
However the key word here is purchased. I definitely purchased Good Notes before (maybe it was version 4 I cant remember) but I can not see a way to revert. The app it self is no longer on my iPad
Absolutely this, was considering an upgrade, no longer.
Removing Family sharing from version 6. They lost me. I opted for a refund and falling back to Version 5. It’s not for me anymore.
Goodnotes v6 shows as supports family sharing in the UK app store.
This is because they made a bit of a mess as the app in the App Store is both GoodNotes 5 and 6, I suppose not to lose their reviews and ratings in the store. Once you upgrade to 6, you have to create a Goodnotes account in the app itself, and this is probably where family sharing then breaks once you’re on Goodnotes 6.
Thus reminding us that a lot of the versioning/upgrade-path/subscription mess we see in today’s app scene is due to functional limitations in the way Apple designed the App Store. I think the phone app era would have unfolded very differently if they’d added a way for developers to charge for major version updates and include multiple versions at once under the same listing.
But Apple went full-in on the subscription approach, and here we are.
The AppStore is the reason I support so few iOS developers. I buy NO macOS apps from Apple’s Store. If macOS devs want my money, they need a website and a download/pay button.
I expect apps for the Vision Pro will be App Store only. And possibly subscription only.
I’m not finding their FAQs very clear 
Does the subscription and one-off price cover MacOS as well as iOS? I had vaguely thought I paid separately for GoodNotes 5 on different devices, but I might be remembering wrong.
I have no desire to upgrade on iPad, but on MacOS for the last year or so I’ve found the app a little buggy (e.g. hanging a little, the left-hand navigation jumping in large notebooks, taking a while to sync).
Not sure I use it on MacOS enough to justify upgrading, and it seems weird to upgrade on one device and not the others. (Mind you, I partly don’t use it so much on MacOS because I found the app a bit buggy, so maybe I would use it more if it worked nicely. I mostly just use it for quickly adding an image or grabbing a note. Heavier work is done on iPad.)
Yes - it is a universal app. If you find that V5 suits you why not keep it? I did upgrade, but I got a free upgrade to V5 and paid 99p for V4 - I thought I owed to the dev! I don’t consider £10 a year a bad deal.
Stay away from upgrading. Their in app purchases are flawed. I ended up paying the full price even though I was having a Life time GN5. The discount never got applied. The FAQ says ask for refund from Apple. It gets denied as it wil I’ll not meet their refund requirements. I’m fine letting it go but not everyone thinks this way and people have started to post reviews as Scammers already exactly for the same reason.
If you have a Lifetime license it means you already paid for the product. Subsequent upgrades should be substantially discounted and not charging arm and length as if it were a new product. In this case when you already paid for GN5 as Lifetime charging again with 3 times the cost of subscription for GN6 negates the Lifetime. It’s not Lifetime of the Product. It’s lifetime of the Version of the Product.
You are actually losing features and paying more in this upgrade. Stay away.
I have purchased many lifetime software and they don’t charge for any future releases. There is a Tip Jar. You have already paid 3 times the price in advance. You should not be paying for every version if it’s Lifetime. Not at this model implied.
This was a dirty move from GN5 to GN6.
My 2cents.
I still purchased a Lifetime GN6 and I still rant about this.
Wow - I had missed those reviews - a lot of anger (UK page). I am slightly peeved that they did not keep GN5 around. I also still use Noteshelf - which has better handwriting in my opinion.
Nevertheless I don’t think that £10 per annum is too bad ( and I have a lot of annotated PDFs in GN). However, I really sympathise with those that thought they had bought a lifetime app.
EDIT: I am not seeing any real advantage to 6, so I might downgrade back to 5 at the end of my trial.
As long as Apple dictates how the AppStore works and as long as everybody HAS to use the AppStore to get apps on the iPhone or the iPad, I am 100% sure that whatever a developer does will lead to frustration with customers when a new version is released or the payment model is changed and the developer does not want to lose customers in the process.
I think that this developer has done quite a good job to be honest:
I am pleased. 
Thank you for pointing me to the 1-star reviews. It encouraged me to write a 5-star review.
This is the problem. Which is flawed and does not work. He did mention that there is a discount and people were charged the full price. The refund process is not working with Apple Store.
Other points you mentioned are all valid.
