Add custom elements to TYPO3 as hidden by default? - typo3

Question

I have an installation with multiple websites. Each site has a site package with custom fields and content elements that are specific to that site.
But the custom fields and content elements are shown on all sites.
In tt_content.php I add a custom element to the type dropdown. How can I make it hidden, then enable it in ts config for the page tree that it is used for?
\TYPO3\CMS\Core\Utility\ExtensionManagementUtility::addTcaSelectItem(
'tt_content',
'CType',
[
'Banner',
'my_extension_banner',
'EXT:core/Resources/Public/Icons/T3Icons/content/content-image.svg'
],
'textmedia',
'after'
);
Likewise, I have some custom fields added to existing elements. How can I make this field hidden unless specifically enabled by the ts config of the page that it is made for?
\TYPO3\CMS\Core\Utility\ExtensionManagementUtility::addFieldsToPalette(
'tt_content',
'headers',
'--linebreak--,my_extension_myfield',
'after:subheader'
);

Answer 1

After some trial and error, I found that I can remove elements and fields globally by adding this to my ext_localconf.php:
\TYPO3\CMS\Core\Utility\ExtensionManagementUtility::addPageTSConfig('
#Remove Custom Content Elements
TCEFORM.tt_content.CType.removeItems := addToList(my_extension_banner)
#Remove Custom Fields
TCEFORM.tt_content {
my_extension_myfield.disabled = 1
}
');
Then add them in again with my page specific ts config file PageTSConfig.tsconfig
#Add Custom Content Elements
TCEFORM.tt_content.CType.removeItems := removeFromList(my_extension_banner)
#Add Custom Fields
TCEFORM.tt_content {
my_extension_myfield.disabled = 0
}

Answer 2

I don't know if I understand the problem correctly, but you could place Page TsConfig in the root page of any web page and then hide the fields accordingly:
TCEFORM.pages {
subtitle.disabled = 1
}