Description:
This mod lets you capture soldiers above S-rank in the campaign and during freeroam.
There are three files that you can use:
spp_gradual: Normal progression and once you have enough S rank soldiers, S+ rank soldiers will start to appear.
Gather enough S+ and S++ will start to appear.
spp_gradual+: Similar to the one above but there is always a chance for S rank soldiers to spawn (start off at 1% and increases as you level up your base). Bear in mind that, until your int-scope is fully upgraded, S rank and above will show up as "?".
spp_100: Every soldier is S++ making it easy to quickly level up your base

As per fekeleke123's request, there is also a version with a reduced brig conversion time (around 10-15 min). This only works for MB, not FOB.
As per doobNemesis' request, there is also a version where mainly A, A+, and A++ rank soldiers will spawn.

Installation:
Installation is done using SnakeBite and there's a great explanation with pictures on the wiki.
But here's a quick rundown of how to do it:

• Download and unzip this mod
  

• Run SnakeBite
• Click 'Mods'
• Click 'Install .MGSV File(s)'
• Go to where you've saved this mod and choose one of the .mgsv files
• Click 'Continue Installation'
• Done :)
  
  

Explanation:
Here's a short desciption of what the mod does; After extracting the data in the steamapps/common/MGS_TPP/data1 file the content in data1/Assets/tpp/motherbase/script/MbmCommonSetting.lua is changed.

The specific content to change in that file is the rows that look like this:
TppMotherBaseManagement.RegisterStaffBaseRankRange{totalSectionLv=0,g=10,f=90,e=0,d=0,c=0,b=0,a=0,s=0,sp=0,spp=0}

To look like this:
TppMotherBaseManagement.RegisterStaffBaseRankRange{totalSectionLv=0,g=0,f=0,e=0,d=0,c=0,b=0,a=0,s=0,sp=0,spp=100}

totalSectionLv: the sum of the levels of all your motherbase sections (in this example 0)

The letters g to spp is the level of soldiers that can appear. These values must sum up to 100.
g: E rank
c: A rank
a: A++ rank
spp: S++ rank


Credits
This is an updated version of Jarhad's updated version of Rawrkanos' S plus plus Soldier Mod as that one didn't seem to work for me on 1.15.3.