Most NSFW games are made with VN software like Ren'Py, which is free. But if you want to use more than text and pictures and want an RPG game but can't afford RPG Maker, OHRRPGCE is very similar and free. You can do most pixel art in that program, but title screens and battle backgrounds will have to be made with other programs like Paint, and scripts will need to be done with a text editor. Most tips can be found on the official website.
https://rpg.hamsterrepublic.com/ohrrpgce/Downloads
If you want to make a 2D platformer like Limbo, Godot or GDevelop would be a good bet (and also both are free). I have more experience in Godot, but still have a long way to go to make a decent game. I just used YouTube tutorials to get my Kanji game in the shape it's in now, I hope these help.
https://www.youtube.com/playlist?list=PL9FzW-m48fn2SlrW0KoLT4n5egNdX-W9a
https://www.youtube.com/playlist?list=PLyckz_-Rzq6ClGevL2fneJ5YJnMPKWa4M
(There are a lot more videos, but I didn't want to post too many on here.)
I don't know what "NSFW Plugins" are, I assume these are used by devs to add/restore NSFW content not allowed on Steam versions?