So far, I found a few places I can't copy the links from:

• Pieces, sets and weapon enchantement appearances from the transmog collection or transnsmong menu
• Mounts and toys from collection, panel or anywhere
• Any sort of item, quest, achievement, toy, mount or npc links in the chat
• Player characters from chat
• Dungeons and raids from the Adventure Guide
• Bosses, NPCs and some spells from the adventure guide
• Zones, raids and dungeons from the map
• Player Characters with display names changed by addon (Link Total RP 3)
• Objects such as bonfires and Anvils
• Quests from NPC dialogues

If I find any more I will update the thread.

Thank you again for making the addon available to the public, hope it can be improved even more!

Hello, thanks for this feedback! I spent a little time on this and was able to get some of these added. I am not a WoW addon expert, I started maintaining this because I use the addon myself and didn't want it to die, so it's very possible that some of my findings here are incorrect. It's the best I've got though after spending a bunch of time on it.

• Pieces, sets and weapon enchantment appearances from the transmog collection or transmog menu
  • I fixed the links for individual pieces in both the Items and Sets views for 2.15, but I don't believe the actual sets have Wowhead pages that can be linked to. There are transmog set pages for each difficulty (the Nathria mythic leather set is here, for example), but as far as I can tell that's an ID that's unique to Wowhead, it's not used in-game.
  • Weapon enchantments are linked in a slightly different way than the others, I'm still working on figuring them out.
• Mounts and toys from collection, panel or anywhere
  • These should work in 2.15.
• Any sort of item, quest, achievement, toy, mount or NPC links in the chat
  • Unfortunately I don't think it's possible to pull them from chat, I spent a decent amount of time on that but the IDs/methods just don't seem to exist when mousing over the item. The mouseover target is the entire chat message, not the specific item, and it's missing the relevant info to pull the spell/item ID out for linking.
• Player characters from chat
  • Same problem as the previous point.
• Dungeons and raids from the Adventure Guide
  • I can't find a way to make this work. The dungeon journal uses different internal IDs for its buttons/links, and there doesn't seem to be a way to convert from the instance ID to the zone ID (which is what I'd have to link to on Wowhead).
• Bosses, NPCs, and some spells from the adventure guide
  • Similarly to the previous point, bosses use their own internal IDs for the dungeon journal and there doesn't seem to be a way to get the NPC ID from those internal IDs.
  • Loot is working in the dungeon journal, though I'm not sure if it already did or if my tweaks for 2.15 made it work.
• Zones, raids, and dungeons from the map
  • I can easily get the mapID associated with the map, but I haven't been able to find a way to convert that to the ID used by Wowhead (the AreaTableId in this DB).
• Player Characters with display names changed by addon (Link Total RP 3)
  • I'll have to play with this one in the future, as I don't use Total RP 3 or any similar mods. I'll get back to it.
• Objects such as bonfires and Anvils
  • Objects don't provide any identifying info on hover other than their name, so this won't be possible.
• Quests from NPC dialogues
  • These quests also don't provide any info on hover, so I won't be able to grab them.

Thank you for your response and action!
Before, I was using another addon with some similar features - Links in Chat, but it wasn't maintained and died with Dragonflight.
It managed to utilize some of the problematic cases, for example the chat, and dugneons if I remember correctly, though it was using Alt-Click instead of hover.

I'm still a bit early in my know-how to meaningfuly help, but I hope the remains of this addon have some useful snippets of code.

Thank you again, you are doing the titans' work for this community and it will always be appreciated!

Any sort of item, quest, achievement, toy, mount or npc links in the chat

This works if you click the link in the chat so that it opens a tooltip. It doesn't work 100% of the time though, but usually works on the first or second attempt.

But I found another case which would be nice to have support for:
Trading post tasks in the adventure guide (e.g. https://www.wowhead.com/trading-post-activity/118).

@cremor Thanks for the suggestion! I just published 2.15.5 which supports these activities.

Thanks, works fine in the adventure guide. But I noticed a related problem: When you track a trading post task and then use the addon to create a link from the quest tracker, it will create a quest link (leading to either a wrong or not existing quest) instead of a trading post task link.

Thanks, works fine in the adventure guide. But I noticed a related problem: When you track a trading post task and then use the addon to create a link from the quest tracker, it will create a quest link (leading to either a wrong or not existing quest) instead of a trading post task link.

2.15.6 was just published and it fixes this (and tracked profession recipes). Thanks for catching it!