iBooks Author end-of-life
Apple has announced use of iBooks Author end-of life: it will no longer be updated for MacOS releases after 10.15 and it won't be available for new users.
More information can be found here:
Apple suggests moving to Pages for the creation of iBooks, but as far as we know, there is currently no support for custom widgets in iBooks Author, and there has been no communication suggestion that this will be added in the future.
This means it wil not be possible to continue to use html widgets in iBooks...
On the Apple support page linked above, you can send feedback - if custom widget support in pages is something you feel should be added, make sure to provide Apple this feedback.
Existing users can continue using iBooks Author for the time being, but now iBooks Author is gone Kotobee Author has become the main way to create interactive eBooks... Kotobee Author has the advantage that books can be read on multiple platforms.
Where interactive iBooks were tied to Apple's 'books' reader apps, we've only had success using interactive .epubs created with Kotobee Author inside of Kotobee Reader, but Kotobee Author has various other output options. More information on Kotobee can be found on their website.