This is not really global, right?

This is the limit per object modifier stack limit.

So perhaps BKE_gpencil_get_lineart_modifier_limits ?

Same as above. But instead of assign you should use set instead.
The get/set combo is used quite a bit in programming so we should change to that so it is obvious that these to are directly related with each other.
BKE_gpencil_set_lineart_modifier_limits

Add a BLI_assert check here to check if the modifier we pass really is a LineArt modifier.

This name is also not good I think.

To me it seemed like this would check if the cache was populated already or something.

Also I think we can make this a general function, no?
We take the input modifier and use the modifier type from it as the type we are looking for.

Then the function name could be something like:

BKE_gpencil_is_md_first_type_in_stack

Gah... Perhaps a bit overly verbose, I'm open for more naming discussion as I'm not quite happy with what I came up with either.

Used -> Use

No period at the end of the sentence.

Besides that, I also think that this should be a bit more descriptive to that the user can the the description here and know that the result is from the first line art modifier in the stack. So this is per object and not scene global.

As discussed in the chat, this shouldn't be here as it has the potential to break older files if you set this to on when loading them.

Because we can potentially only compute features lines once per modifier stack (Use Cache), we need to have these override values to ensure that we have the data we need is computed and stored in the cache.

Keep a pointer to the render buffer...

You forgot to change global here to modifier

Use cached line art data from the first line art modifier in the stack.
Certain settings will be unavailable when using cached data.

This is a bit of a run on sentence.

I think:

This is just a pointer to LineartCache::chain_data_pool, which acts as a cache for line chains.

A copy of rb->chains so we have that data available after rb has been destroyed.

Hmm, why is this still a TODO shouldn't you know which feature edges you have in the cache?

Shouldn't it just contain the edge types that are in "limit" info data type?

Isn't it better to name it ptr instead of onetime?

The TODO is not relevant any more. Removed.

I think I'll put it "Geometry Processing", it's more aligned with what those options do.

Or "Use Cache" ? Because the user don't care about what's exactly cached anyway?

Then maybe "Use cached calculation" ?

I think the name is ok.

Hmm, I'm also unsure how to name this.
As at a glance all the suggested names doesn't properly convey that the cache is not from the current modifier.

But I can't really come up with something better that is short and to the point.

Maybe we should be more verbose here. What about this:

Only evaluate the scene data once in the first Line Art modifier in the stack. Then reuse that data in this modifier for better performance.

The problem with this description is that the cache does cache for all modifiers, like when the second modifier selects some line types that the first modifier does not select, both line types will get cached, this description sounds like if you don't select all line types in the first modifier it's not gonna be available later on.