When we talk about isekai in modern contexts we do commonly refer to the post 2015 Narou-Isekai where one certainly draws a distinction between isekai before 2015 or after.

The thing about "old" isekai is that they exist more as fantasy stories with specific messages and themes, where Narou-Isekai can be specifically placed in the context of the latest evolution of the Otaku after the failure of sekai-kei and the increasing self-indulgence from CGCDT, Battle-Harems all the way to Isekai.