Hunting Grounds only affects farming, not backyards like apples or veggies (as those are entirely unaffected by fertility) @Anima Mundi.
It's still a decision to make though, because even with bad fertility it's useful to supplement your barley/malt imports with a low own production of barley, and thus keeping costs low. Obviously you can skip that if you have another fertile region producing barley for all of your infertile regions.

And yeah, Hunting Grounds is OP, especially for regular deer regions - as long as you don't do any farming there at all.

Those 66 lambs a year give you not just 66 hides/leather but also 132 meat, or 264 meat per year with the 2x meat perk (as that applies to deer, sheep, pigs and goats, but not trapping). If you have a town with 3 other food sources, you can produce enough meat for 88 families with that alone, so far from useless. And you'll always have deer, likely will add pigsties if you took the double meat perk anyhow, so you can more easily supply even large towns with meat with a minimal amount of labour invested.

The gestation (pregnancy) period is 145 days +- 1 or 2 days or so of variance. All sheep are ewes, so you get 26 lambs from 26 sheep every 145 days, or almost 66 lambs per 365 days. If you have more sheep than that, you reach the hard cap of about 66 lambs per year, because you can never have more than 2 lambs being born within the last 10 days plus the current day (so actually 11 days). The rest of the pregnant ewes will simply remain pregnant until you fall under this hard cap - so they might actually remain pregnant for many more months before the game allows them to birth a new lamb.

This is not the maturation period - iirc they take 1 full year to mature from new born lamb to sheep.

I usually just get 30 sheep, then set all lambs to be slaughtered for meat and hides (they give the same amount as mature sheep for some reason). And 30 sheep times 9 wool per year is way more yarn/clothes than anyone needs anyhow, especially until clothing is actually being consumed over time.

@† N † no, it seems correct. You get 66 lambs per year, lambs are born every 10/11 days. That's the "gestation" period. What OP meant to say there for the 145 days is the maturing time, not gestation.

66 Lambs per year also seems to be the hard cap. I get this is made to limit trade exploitation, but then it makes it useless as a food source (max of 10 meat /month or 20 with skinning perk), while you can still scale up Yarn production and still exploit trade.
Makes this Sheep perk only attractive to fire and forget: get 26 sheep so the village can get a constant 660 silver /year.

This whole mechanic of livestock would be more immersive if it could be a viable food source and require the player balancing pastures with space for crops (or taking perk for Fenced fields), yarn production, and the ratio of slaughter to birth rate. my 2 cents

Curious if the HG policy also affects backyard extensions, haven't seen it mentioned anywhere.
If not this should be even more attractive to any region with bad fert, saving a lot of space on veggies and apples

I don't quite understand how you get 66 lambs a year from 26 sheeps. 26 sheeps give 13 lambs every 145 (+-1) days. Isn't that right?

It's up-to-date.

For those few changes that did happen, I left in the old info with the strikethrough function, then added the corrected version.

Is this guide still relevant or have there been any updates to the persks system?

There's a great new guide about trading if you want to get deeper into it: https://cs2bus.com/sharedfiles/filedetails/?id=3421496024

It also has nice tables showing the trade route prices, making that part easier to understand.

Generally it's 10x the base price of the good for your first route, 20x for the second, 30x for the third etc.
But even if you pay 300 for a trade route that you really need, the perk only saves you 150 silver. The charcoal perk can create that much with just 30 coal produced, about 1/7th of a kilns monthly output with one family working.

It's not that the trade perks are bad per se, it's that you can overproduce so easily that other perks become far more interesting and the money saved on trade routes quickly pales in comparison to how much you can earn through exports.

Maybe I'm wrong in something, but in my recent games my trade routes costs at least 260 - 320 gold, I've never had trade route prices low as 80.
Well I'm still not good in making money in this game, probably I see more benefit of Trade Logistics perk than it actually is.