This bluepost links the article covering Classic Era’s debut:
https://us.forums.blizzard.com/en/wow/t/world-of-warcraft-classic-and-burning-crusade-classic-characters-and-realms/
" WoW Classic will direct you to the new Classic Era realms which will have the latest phase of WoW Classic content in it and will not progress into any WoW expansion content."
The future of Classic Era is unknown. Currently we’re playing Phase 6 but there is nothing to say additional phases will not be developed later on. What is certain is that none of retail’s content will be in Classic WoW.