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I have noted that in books/articles dealing with history, "would have" is often used. E.g.:

Many of the tools used by Woodland people would have been familiar to
their Archaic ancestors.

Such spears (made of yew or spruce) would have been thrusting weapons
not javelins, due to their poor piercing power as a projectile so
would have required the hunters to ambush their prey.

Is this usage related to likelihood/uncertainty? Would it change the meaning if I rephrase the first sentence and used "by Woodland people were likely familiar.."? Or what if I use simply "past simple" tense in the other example?

A historian uses the conditional to denote probability/possibility.

So, this: Many of the tools used by Woodland people would have been familiar to their Archaic ancestors.

can be read as:

It is likely that many of the tools used by Woodland people were familiar to their archaic ancestors.

Would have is a modal verb that is used either:

to talk about things that we imagine happened in the past
or
to talk about things that might have happened in different circumstances.

Your examples are all typical of the first usage. That's to say, the speakers are all describing situations for which there is some evidence but not sufficient for them to speak with certainty.

Once there is proof or, at least, sufficient evidence, speakers simply use the past tense.

So usage of the would have modal tells us that the speakers surmise from the evidence that this is what would have happened.

The second use of would have is for statements in which people describe what might have happened if the circumstances had been different:

If I'd had more money I would have bought the luxury model.

Could have and should have are similar modals generally used to describe things that might have happened but for which there isn't proof.

They could have reached the station by now (possibility)

They should have reached the station by now (expectation)

https://www.perfect-english-grammar.com/could-have-should-have-would-have.html
https://www.espressoenglish.net/past-modals-should-have-could-have-would-have/
https://learnenglish.britishcouncil.org/en/english-grammar/will-have-or-would-have

The use of "conditional / hypothetical / not necessarily true" would have [been] rather than Simple Past were in OP's examples is a kind of linguistic "hedge".

Normally we think of hedges like this as "weakening / softening" the impact of an assertion, but it's not obvious that's the effect in the specific examples cited. The way I see it, the author is implying something along the lines of if you were to study this issue carefully, you would come to the conclusion that [whatever is being asserted in this sentence is true].

I think this leads to the further implication that the author himself has in fact studied the issue in depth, and come to the stated conclusion, so you should believe what he says - because he knows more than you, and is better qualified to exercise judgement, where irrefutable evidence is lacking.

In practice it's unlikely that the reader is going to conduct more research than the author, since this particular style of writing is often favoured by people who are experts in their field. So although we tend to say that hedging weakens an assertion, in certain contexts it actually adds authoritative emphasis. A bit like the way "understatement" often has the net effect of amplifying a point being made, rather than undermining it.

Some academics will doubtless imagine that by saying something would have happened rather than did happen, they're somehow "protecting" themselves from being proved "wrong" if further evidence comes to light showing that whatever they claimed happened could not have been true. Personally, I don't buy that perspective, but I'm sure it's often a significant factor in choice of phrasing. If they'd been honest, they'd have expressed the lack of certainty more explicitly (by saying something might have or probably happened, for example.