Honest question: Why would you want ligatures in plaintext? Especially in code, wouldn't that be weird as some people tend to manually align their code? Like,

  modular_raft = 10
  spam         = 20

The "ft" in modular_raft should convert to a ligature so (I assume) it will then take one character, instead of two. Then developers viewing that source would have different alignments, depending on whether they use Sublime 3 or not.

(EDIT: Formatting.)

Some people use monospace fonts with ligatures specific to common programming languages: https://github.com/tonsky/FiraCode is one such example.