Since iOS 16.4 and iPadOS 16.4, Apple has made notable changes to how Developer Beta and Public Beta updates are downloaded on iPhones and iPads.
Instead of using beta profile files that could be downloaded onto your device as before (the files that you downloaded from this website), you will now have to use an Apple ID that participated in the Apple Developer Program (for Developer betas) or the Apple Beta Software Program (for Public Betas).
The Apple Developer Program is not free, you have to pay $99/year to join. The Apple Beta Software Program, however, is free.
As their name suggests, the Developer beta are intended for developers to test their apps before releasing them to the public. The Developer beta is usually released a few days earlier than the Public beta. However, the first Developer beta version is usually released immediately after the opening keynote of Apple's annual WWDC (in early June), while the first Public beta version is typically released about a month later (usually in July).
Because it is released earlier, many people prefer to install and use the Developer Beta. But, it also carries more risks than the Public Beta.
To continue using beta software on your device in the future, you should join one of the two programs mentioned above because Apple will no longer release beta profile files for iOS 17 and iPadOS 17 and later.
Scroll down to the bottom of this page to see the next steps if you are already a member of the Apple Beta Software Program or the Apple Developer Program.
Sign in with your Apple ID
To join the Apple Beta Software Program, please visit beta.apple.com. Tap on Sign up button, then log in with your Apple ID.
You can use a different Apple ID to join, it doesn't have to be the Apple ID you're signed in with on your device.

Agree to the Terms
Tap on Accept button to agree with the terms and conditions.

What's next?
After registering for the Apple Beta Software Program, you need to enable the "Beta Updates" option in the Settings app to receive the latest beta updates.
Learn how to enable iOS / iPadOS betas from the Settings app →Sign in with your Apple ID
Go to developer.apple.com/programs/enroll and click on the Start Your Enrollment button, then log in to your Apple ID.
You can use a different Apple ID to join, it doesn't have to be the Apple ID you're signed in with on your device.

Confirm your personal infomation
Enter your personal information for confirmation, then click Continue.

Select your entity type
Select the entity type, I recommend selecting Individual / Sole Proprietor, then click Continue.

Agree to the Terms
Click the checkbox to agree to the terms and services, then click Continue.

Pay for Apple
Click the Purchase button, then enter your payment information. Apple accepts Visa, Mastercard, Discover, and American Express.

After completing the payment, your account will be reviewed by Apple's team, which may take 1-3 days to complete. You will receive an email notification if your account is ready.

What's next?
After registering for the Apple Developer Program, you need to enable the "Beta Updates" option in the Settings app to receive the latest beta updates.
Learn how to enable iOS / iPadOS betas from the Settings app →