Socketed items: Which "pre-installed" skills can they have?

@Queegon wrote this somewhere else:

Werebear (or other lvl 6 skills in general) can only show up on items of ilvl up to 36 but for the base to get 3 sockets as well it has to be ilvl 26 and higher, which leaves you with very limited farming areas of late normal and early nightmare.

Is there an overview / guide about this? 20 minutes googling didn't help me. As in: It is possible (to stay within the example) to find, for example, a +3

Werebear (lvl 6 skill) -and- +3 Fireclaws (lvl18 skill) 3-socketed druid pelt? If yes, in which area levels? etc.

I always thought there was no upper area level limit to equipment, as in "in high area levels, you can find ALL the stuff including the lowest-level items, so although the pool of possible drops is larger, at least it includes everything". But according to the above, it seems like some configurations of socketed items cannot drop everywhere.

TL;DR +

Werebear +

Fire Claws can exist. If you want it to have 3 sockets as well, ilvl 26-36 is your range. And you can see the chance of lvl 6 skills spawning is just 10%.

https://maxroll.gg/d2/resources/staff-mods

namely
Tier 1 - Level 1 skills
Tier 2 - Level 6 skills
Tier 3 - Level 12 skills
Tier 4 - Level 18 skills
Tier 5 - Level 24 skills
Tier 6 - Level 30 skills

Wouldn't TL;DR on a good answer Thanks!