In add mode, it's the whiteness of the texture, not opacity which tells how bright it is. In the multiply part, the texture is really plain, but I did show it. It's just completely white which sharply fades to black. Only the black part matters as that's what creates the shade and white is ignored.

But thanks for your responses, I'm interested in seeing if you will make a video on this for 2.8.

Here's how you do it:
0:12 - Shaderless = Emission node set to 1
0:20 - Matcap rim light = Layer Weight node set to mix add
0:40 - Mix rgb (Add mode) on the base material. Oppacity in that node is = Layer mix factor
From 1:16 to 1:27 - You didn´t show what the matcap mas was like. Please re-record the video showing what those maps look like.

Great news is: Yes, all of that can be done in 2.82 with nodes.
And thank you for showing me these parameters. Now I see what you'd like to accomplish.
Please let me know.
Thanks!

If you can get a "multiply" texture and an "add" texture work at the same time in nodes, so that they also play off of a normal map texture and are properly overlaid on whatever other shader in Blender 2.8, that I'd be interested in knowing about.

In 2.79 it's as simple as applying the diffuse, matcap and normal textures one after another within a material in Blender Render mode and it just works, but I don't know how to do that in nodes.

Short example, given Steam lets through a link: vimeo.com/405547694

Ah, yes, I've seen those 2 color 16x16 bitmaps in mmd. The matcap technique the Zbrushers use is something I've been looking forward to do a tutorial on. But you're right, maybe I should publish WHY the MMD uses these resources on their shading, and the appropriate method in Blender 2.83. Thanks!

Interesting method! You can also get pretty nice effects which fit the anime style by using multiply and additive textures, which are mapped along the model's normals instead of UVs. Kind of like matcaps, but just to add bright spots (speculars) or shadows with gradients. MMD models and anime models in the VRChat domain use these to great effect. Also they combo well with models with more detail through normal maps.

I couldn't really figure out how to setup them in 2.8, so I'm still sticking to 2.79 solely because of them, for the anime style.

This is not "the light theme". This is called the XSIMOD theme, which I mantain in my {LIEN SUPPRIMÉ}gumroad.com/3dcinetv

I got one:
why do you use light theme?

If you got any questions regarding the assets or the shader, please ask me about them here in the comments. Thank you for checking this out! :)