You only have a few months to finish up GTA Online if you're on the previous consoles

Title You only have a few months to finish up GTA Online if you're on the previous consoles
Author Nick Diamon
Posted in News
When June 16, 2021

Rockstar Games is ending support for GTA Online on the Xbox 360 and the PlayStation 3..

Read the full article

Not that I really plan to, but can one play the 360 version of GTA V on the Series X (I kinda doubt it, since the XBone got its own version as I recall)?

Your recollection is correct. The Xbox One/PS4 builds are the most recent console versions, so there is no BC forwarding for 360 or PS3 versions.