To correct your original post regards Quartz.
It has been updated to both work in WoW Classic Era and Wow Retail
https://www.wowace.com/projects/quartz/files
3.7.7 for WoW 1.15.0 updated November 2023 ( WoW Classic Era Season of Discovery )
3.7.6 for WoW 10.1.5 updated July 2023 (WoW Retail Dragonflight )
3.6.12 for WoW 3.4.0 updated August 2022 ( WoW Classic WotLK )
If you are using one of the official servers then one of those will hopefully work.
And in regards to access to API help the most up to date site is
https://warcraft.wiki.gg/wiki/World_of_Warcraft_API. All official servers are pretty much on the same API base, with just game functionality changing according to which version it is. Wow Classic Era (Season of ... ), Wow Classic ( WotLK ) and Retail ( Dragonflight ). UnitSpellHaste is no longer a function on the official servers - they replaced the API a few expansions back and all versions now use the same version for the most part.
So, as per Fizzle, we can't help if you are not working on an addon for official versions of the game. If you are working on an addon for one or more of the official versions. Try the Quartz versions I mentioned above.
If you are planning on an addon for the next Classic version ( which will be Cataclysm ), due to arrive in the next few months. Then, going back to the old Cataclysm version will not help, as most if not all of the functionality will have been replaced with new ones a la the wiki link above. When Cata Beta or PTR hits ( assuming it does ) you can try one of the versions of Quartz as I listed and linked above to see which one is likely to work the best ( probably 10.1.5 version until they make a new version for Cata Classic )