The pictures shown during the keynote show subtasks. Hopefully this is something that Goodtask can build upon.
Unfortunately API has not changed at all. It seems like subtasks are supported as 1-level deep reminder but not readable at all in API level.
Also image attachments and URL are using its own that can't be read.
I'm not sure it'll change along the beta period or not but I'll try to ask for it.
There also seems like some number of compatibility issues with no-time tasks that's made in new Reminders app. I'm not sure why they did it this way but they're saving Jun 4th date value as tomorrow rather than today (today is Jun 4th). It's identical on iOS and macOS so they might be intending it rather than making a mistake. Will need some time and some communication to sort things out. 
Goodtask is not working at all on iPadOS. Currently all lists are out of order and all lists have big 0 next to them. Installing now on iPhone so will find out on there, have not upgraded my Reminders to new Reminders yet, upgrade button still showing in new Reminders app because haven't finished installing betas yet.
Also will update this post when I install macOS Catalina on my macBook 12, so will let you know if that messes up.
Reminders asks you to update your iCloud database when you open it, you have option to skip for now and use old database, but upgrade button will stay there.
UPDATE: Tried uninstalling and reinstalling app, same result, just nothing...
Thanks for the info, @sparkktv.
I've already checked in iOS 13, iPadOS 13 and macOS Catalina.
Current status is like this..
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When you upgrade your Reminders database, they will sync by themselves and previous one will sync with the ones on previous versions. It works like how iCloud Documents worked few years ago when iCloud Drive was introduced. They're separate databases.
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Database sync between new OS's but GoodTask has some issues with the new APIs. Newly added features are not managed well. List will be reset since database has been reset and lists are covered as new lists. EventKit API seems to have bugs as changes are not reflected well. Need to re-open list to fetch the changes.
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Couldn’t check if changes work correctly if you don’t upgrade your database. I’ll check with other account later today. Even if they sync, it doesn’t seem to work correctly due to lack of notification that doesn’t fire when changes occur.
Yes I see that, even tho the lists say 0, if you open them one by one, they show the tasks. But Smart Lists do not work including the Today list.
For now I'm just using Reminders, it still works fully the new version even without upgrading the database. It just keeps the blue upgrade button there.
Thanks for taking one for the team and giving it an early peak. I'm leaving on a business trip tomorrow and want both my devices available so I'll have to defer any updates until later in the month.
No Beta Profiles, so it has to be done old way via iTunes on Mojave but Xcode 11 Beta must be installed or using Finder on macOS Catalina.
Well, that will definitely keep me from changing my mind till I get back home. Could take the Mac along, but I've enough technology in the bag. Nothing pressing I need to start playing with in it yet anyway.
I'll just say other than Goodtask not working & my bank app crashing, it's the most stable betas I've ever used from Dev beta 1. I'm shocked with the whole warning from Apple about installing on non Dev devices. But smooth as smooth can be. Face ID is way faster. All apps are working except the two mentioned above. Updating apps is different now, you click your profile picture in the App Store to find App Updates, there is no more tab.
Had some issues with watchOS 6, but they fixed themselves after a few hours. The watch app on the phone becomes very useless, had to reinstall some apps thru the new App Store on the watch itself, that's where the watch app on the phone becomes useless because it claimed installing but did nothing unless I installed them thru the watch's App Store on the watch itself. Again very stable once problems were fixed which were likely sync issues.
macOS Catalina has the most app issues but again very stable and works great. Goodtask does not work correctly (same as iOS issue) and other apps like OneDrive, Box & Dropbox just crash out, it seems to be a folder thing ( how you set up a sync folder for them) it won't read the folder. You use Finder now when you plug in an iOS or iPadOS device. Apple Music, Apple TV & Podcasts took some time to sync. Again though very stable, I would install it on my main Mac but can't due to need of Xcode for app development and not being allowed to use beta to update apps.
Overall I would say all are ready to be daily drivers which is shocking, but last year was same with beta 1 and each beta got worse in stability after that so we shall see. I'm waiting on my order of the Lightning to USB Camera kit so I can plug in hard drives and also set-up my Magic Mouse on the iPad Pro, The Verge claims it works Bluetooth also but have to set it up through direct wire like on the Mac.
I made some more observations.
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If you don't upgrade your database, it seems to sync fine with old devices but the Reminders lists may lose the colors.
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GoodTask should normally work but 'Refresh' does not work as mentioned above. You need to manually go out and go back into the list and count numbers on the lists will not be changed accordingly.
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If Scheduled smart lists doesn't work, you need to turn off 'Use Start Dates' or check 'Due date' option inside the filter settings.
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Checking off 'Reminders' named Reminders list inside 'Settings - Lists & Calendars' may help if data is not loading properly.
@sparkktv, adjusted topic title to make it broader. 