If you want raw gold/vendor farming, as mentioned, WoD and MoP instances are good for that (‘good’ as far as raw gold goes, it’s still chump change compared to advanced goldmaking methods that make use of the AH).
Legion materials still sell decently well on the Auction House on large realms and are very easy to farm up due to the abundance of nodes and access to stirrups. You will need Legion flying to make the most out of it though. You can also incorporate Blood of Sargeras world quests and Boon of the Bloodhunter farming into that since Blood, while Bound of Pickup, can be traded sellable raw materials in Dalaran. As an advanced method for Blacksmiths, you can get the plans for a sellable mount in the Nighthold raid and make that with your Blood of Sargeras, it still sells rather well. Other profession crafts from Legion that still sell well include the Enchanting pets and Inscription Glyphs, though Legion professions do require quite a bit of setup since the most valuable plans require both high profession skill and the completion of lenghty questlines.
Going further back, a number of Pandaria crafts, most notably mounts from Engineering, still sell rather well and the materials can be obtained with minimal effort from the Tillers Farm. The specifics take a bit to explain so your best bet would be to google search some guides on Sky Golems, Pandaria daily profession cooldowns, Pandaria crafted transmog and the Tillers Farm. The added bonus of the farm is that it is relatively easy to set up (takes a week or two of low-effort dailies), that setup can be done while levelling, once set up, it only takes a couple minutes every day to maintain and reap the benefits and there are no diminishing returns from having it on multiple characters, so an army of level 90+ alts can net you quite a bit of stuff quite literally just for logging in and clicking a couple plants.
That being said… The best and easiest gold is always in the most recent content. Yes, you can make some gold in Legion and WoD or Pandaria profession stuff… but investing the same amount of time into goldmaking strategies involving current content will result in loads more gold. And even within the current expansion it pays to stay current with the new content releases and market changes. The market changes all the time, the thing that made ridiculous money a year ago is completely different from the most profitable thing from two months ago and will be different from the thing that’s best today. For example, when the bee mount was added in a new patch, for the first couple weeks you could make literally millions farming up jelly to sell on the AH. Now… well, you can sell it, but don’t expect breaking the bank with it. Or when BfA first released, before the first raid opened up, you could make millions with Inscription trinkets or 350 BoE items farmed up from random mobs in the overworld. Those are now worth next to nothing. When Nazjatar first released, I made a couple mil just skinning Snapdragons that people were killing for their daily quests. Those same skins are now something like 3g each and not worth the time to fly over to the spawn area. When Nyalotha opened, people were selling corrupted BoE items from raid trash for multiple millions EACH. That market has crashed multiple times since then.
Long story short… if you want to make a couple tens of thousand gold to cover repair costs and whatnot, your best bet is to just do some low effort raw gold farms or material gathering. If you want to make hundreds of thousands of gold or your first mil… You should look into profession stuff and figure out what to make, how to setup making it and how to make it for the least amount of money/time invested for the biggest return. If you want to make millions of gold… you need a bit of investment capital, you need to stay current with the game and you need to be agile and flexible and react to new content releases and new markets.
Pretty much just like IRL - you can get any dull job if you only want enough money to pay the rent. But to go beyond that, you need to work out a plan.