Kaivax, a WoW forum community manager, revealed that WoW Classic's class design, battleground mechanics and stats on existing items will be set to their 1.12 state, despite the game releasing content that expands beyond that. This removes "progressive itemization," so if the stats on a specific piece of equipment was changed during the original updates, that won't take effect in this version.

You can’t go wrong with a mage. In fact, Mage was the most popular class in World of Warcraft: Classic due to their ability to farm with ease. Back in the day, the player majority even leveled mages as alts in order to gather gold quickly. In addition to their monstrous ranged DPS, mages also have tons of useful utility. As a mage, you’ll be able to conjure food and water, place portals to the major cities, and even earn gold from doing so.
A demo of the game was available at BlizzCon 2018, and was downloadable on home computers for anyone who purchased a BlizzCon ticket or virtual ticket. The servers became available when Opening Ceremony started at BlizzCon 2018 and was set to end on November 8, but was extended until November 12.[4] Players started at level 15, and the only available zones were Westfall and the Barrens.[5] It was based on patch 1.13.0, essentially patch 1.12.0 ported to a modern infrastructure. The first day of the demo, there was a playtime limit of a cumulative 60 minutes with a cooldown of 90 minutes, applied through the BlizzCon Exhaustion debuff.[6] The debuff was removed on the second day of BlizzCon 2018.[7]
To avoid this, the team “taught” our tools some new tricks so that we could update WoW Classic without affecting the current version of the game. We copied the 1.12 data into a new project, taught the tools to distinguish between them, and now as the WoW Classic team makes patches, any changes stay within the Classic chain without ever interacting with the current game’s data. This might seem simple—like copying a directory—but the tricky part is teaching all our tools how to understand this so that they can make edits automatically. Being able to use our internal tools is a huge benefit, and we wanted to make sure Classic had access to all of WoW’s infrastructure and data.
Another hurdle we had to overcome was how to store and merge our data. World of Warcraft has multiple updates or patches in development at any one time, each in a different stage of development. If an art asset or terrain file is added to one patch, the system is designed so that it also automatically shows up in later patches. This meant that If we had simply inserted a new patch for WoW Classic into the current development environment, we would have overwritten things like the current broken dam in Loch Modan with the previous intact version—and as you can imagine, this would have caused issues for Battle for Azeroth.