After using Joomla 4 for a recent project, and reading your upgrade notes for Joomla 4 and modules, I can highly recommend and request, please please upgrade this plugin to Joomla 4 compatible, even if you have to start charging for it.
The Joomla 4 plugin for email protecting is very ineffective still after all these years, it doesn't grab text from all other components, only content articles from Joomla and K2 directly.
Your plugin was SO efficient at grabbing any email it found and protecting it, that it grabbed emails from other components like Galleries and SPPageBuilder and many others, VirtueMart, all automatically. Right now Joomlas new plugin won't grab emails from PageBuilder, which means this new project isn't protected.
I would be willing to pay for this plugin going forward if you could make it compatible with v4. Even after all this time Joomla has not figured out this is a need, and they need to make it happen. If you can make it happen with your 3rd party plugin they should be able to.
I am also very interested in you releasing a new version of the Email Protector for Joomla 4.
I recently updated from Joomla 3 to Joomla 4, using Helix Ultimate template. The email cloaking does not work with the content displayed by this template. It is ridiculous. It also didn't work before with Joomla 3. I was really grateful when I found your Email protector extension.
I kindly ask you to make a version for Joomla 4. Of course I would also pay for it. I do not see another reasonable solution to cloak the emails without deinstalling the Helix ultimate template.
I just suggested the issue should get reported. Seems like nobody did. Nobody claimed they did.
Answering your other 2 questions:
However, it is still a content plugin. That means it only works in content specifically passed through the content plugins. That is only the case with some components and some modules. So the core email cloaking won't work in other components and modules. And not in email addresses you might place directly into the templates index.php file.
So that was the reason to port Email Protector to Joomla 4, as that DOES work in the entire HTML output of the site.