Please create issues for REI Plugin Compatibilities here, this external mod is not a part of the REI base mod.
REI is MIT, REI Plugin Compatibilities is not, the license is shown here or are attached with every jar with its LICENSE file.
If you have a valid reason for viewing the source of REI Plugin Compatibilities, the request will be reviewed on a case by case basis.
If approved, the sources provided do not provide you with any legal rights to any intellectual property with REI Plugin Compatibilities, and you may only use the information for your internal, reference purposes.
If you wish to learn the REI API, check out the REI wiki, or ask about in shedaniel's discord.