Achievements were put in place at the very very end of TBC (the patch leading into Wrath) and updated graphics were even later. I think you’re safe. 
Well enough people pretty much responded explaining everything I would guess about a possible TBC classic release. A year from Shadowlands release makes more sense then as soon as Classic players are bored and unsubbing. They know the diehards will stay subbed playing classic and are just hoping some shell out the full price for the new retail expansion.
Blizzard as a company doesn’t want to divide their playerbase up and kill every version of WoW which is why they stagger everything. Just look at BWL and AQ. They were released at times when Retail was going through lulls. The same thing will happen with Naxx and TBC if/when it is released.
If SL is a November release, Naxx will likely release right before Xmas, or more likely in January after the break. If they decide to do a TBCC the release probably depends on how well SL sales/sub numbers are but November 2021 will be the earliest anyone can hope for. I wish I could RemindMe! One Year “say I told you so”, on this thread because I’m about 99% sure this would be their plan for TBC servers unless SL is an even bigger failure than BFA.
BGs are better in TBC so realm transfer would be soooo great.
I hate leveling toons
Blizzard is already moving up their heroic->mythic->lfr raid timelines for shadowlands. Which could potentially mean a larger “drought” between the second and third raid dungeon in SL making space for a TBC release.
I’m kind of hoping they don’t do Naxx for 6 months though. People do not need their entire raids to be full T3 geared before TBC releases. Even cutting AQ40 back to 4-5 months and having Naxx be similar would be ideal.
I wouldn’t be surprised if tbc was somehow included in the actual retail game. This would prevent splitting the player base and get ppl inadvertently playing retail.
If players are ready for more Classic content, the answer may have to be, " Too bad."
If the answer is to keep releasing the previous expansions, we would end up back on Current.
It seems it takes the “classic” out of Classic.
Did not read every reply, but I think it makes sense to keep current servers progressing and allowing people to copy their entire character list to new vanilla servers. I predict that more people will want to play BC, so they wont need to have as many vanilla servers anymore.