So there is no new idiomatic way to clear all cache files.
Twig project maintainers leave it for library users on their own. The arguments are:
The argument was that Twig itself will only support filesystem cache (due to opcache). Thus clear() would be the same as removing the cache folder manually. Thus it doesn't need to be in the interface. So you have to clear your cache either manually or write the method yourself.
More details may be read in relevant issue early referred by @alain-tiemblo in comments.