I don’t know if there is 1 standard (already), but I know that XML and CSV are quite commonly used export and import formats. And good import and / or export tools usually allow attributing certain columns of an old system to that of a new system (for ex. the article title from Grav to the article title field of Wordpress, and vice versa).
I guess Wordpress is a good reference regarding import and export tools and standards (see for ex. Tools: Import and Export | Learn WordPress). The Wordpress maker not too long ago spoke about its new approach of “data liberation” (you can see that part of the speech here: https://youtu.be/c7M4mBVgP3Y?si=s8F-xwHIVr9w8xZn&t=2064), so I think they’re also still in the beginning and learning process regarding interoperability and transferability.