No, this is a huge misconception.
All the old content will still have it's level bracket, you will be able to choose one expansion for leveling and tune it appropriately, but otherwise the levels will still stop scaling to the expansion level. Raids won't be scaled down or anything of the sort. So if a raid was previously 100 lvl (wod) it will be a level 30ish raid in SL, and a 110 lvl raid will be like a 35 lvl raid.
https://www.mmo-champion.com/content...oper-Interview
If you aren't in Chromie time, zones will only scale up to the cap for that expansion. You can still level that way though!
If you want to go back to Pandaria and farm for transmog or rares, you wouldn't want the zone scaling, so you need Chromie Time to scale it.