Floating above the Plaguelands, the necropolis known as Naxxramas serves as the seat of one of the Lich King’s most powerful officers, the dreaded lich Kel’Thuzad. Horrors of the past and new terrors yet to be unleashed are gathering inside the necropolis as the Lich King’s servants prepare their assault. The Scourge marches again…
Good luck sweaty dudes who aren’t at work!
There’s also a guide on Barrens Chat, don’t see one on Icy Veins yet though. Not sure why they only ever mention Wowhead when there are other sites just as dedicated out there.
barrens.chat/content/wow-classic/wow-classic-naxxramas/
The only reason why I am not playing Shadowlands right now is to get my Tabards for my classic characters and the set pieces 
Do items gained in classic sync up with mog libraries in “retail”?
If only I could take my T3 transmog to my retail account
No. The games are completely separate, aside from the fact that you can buy the subscription with retail gold via the token.