Possible to scrape the date a website was last modified?

Im looking to scrape a list of wordpress websites for the date they were last modified.

Some plugins will publish a "dateModified" in the source code but I find not very many sites have this. I've been manually collecting this and it takes forever.

Its possible to display the last update on sites I own using:

<?php echo get_the_modified_time(); ?>

Im wondering if its possible to somehow display this info on the front end of any public site.

Lots of sites include ld+json schema; perhaps look at scraping the modified datetime from there.

You can try looking at the Last-Modified HTTP header, though this open to manipulation so may not be useful for critical applications.
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Last-Modified