General purpose pixelart tutorial by Derek Yu, Spelunky’s designer
https://www.derekyu.com/makegames/pixelart.html
General purpose tutorial in forum thread format by @curepixel
https://pixeljoint.com/forum/forum_posts.asp?TID=11299
Comprehensive step-by-step classic top-down RPG pixelart tutorial by Jason Perry
https://finalbossblues.com/pixel-tutorials/
Specific-minded bite-sized tutorials by a pixelartist who worked on Celeste
https://saint11.org/blog/pixel-art-tutorials/
Pixelart tutorials and blog posts by @thisislux
https://twitter.com/i/events/1488167866188763149
(PAID) Pixelart comprehensive digital book