Yes. For Quick Blocks and Focus Sessions, add the website itself to your folder as well as the app. Website blocking works in Safari.
Add a website to a block
Open the folder you use for the block, or make a new one (see App Folders).
In the app picker, add the website as well as the app.
Save the folder, then start your block.
A Focus Session you've already set up keeps the apps it had. To add the website to it, edit the session and choose the folder again.
Blocking an app doesn't block its website. To your iPhone, the Instagram app and instagram.com are two separate things, so add both if you use both.
Other browsers and in-app browsers
Website blocking comes from Apple's Screen Time. Other browsers, and links that open inside apps like Instagram or X, may not respect it.
If you find yourself switching to another browser or app during a block, add that app to your block too.
App Limits
App Limits are for apps only, so you can't add websites to them. iPhone can't measure time on each website in a way we can use.