It is not a retail progression server. Era is also not #NoChanges. Its latest phase is free to be developed with any kind of novel Classic WoW content additions the devs see fit, so long as retail’s content is excluded.
Linked is the article Blizzard put out for 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."