PHP 8 uses T_NAME_QUALIFIED as the token type for symbols like Foo\Bar (previously, each section of an imported symbol appeared as a T_STRING separated by T_NS_SEPARATOR but now it appears that the entire symbol is one token). According to release notes, this was implemented into phpcs as of 3.5.1. There are also T_NAME_FULLY_QUALIFIED, and T_NAME_RELATIVE. The RFC proposal explains it pretty well and you can see the actual tokens listed on the PHP tokens page

As a result, this sniff probably won't work to find imports any more until it can be updated to detect these new tokens and parse them correctly.