First off, thank you Obsidian for even allowing the option to enable/disable ligatures in game (they are found in Graphic options, one of the check boxes). It shows a commitment to detailing, even if it would probably be considered of negligible importance to most...
I would hazard a guess that most aren't familiar with ligatures--it's a detailing thing in the world of graphic design and typography. Here is a very succinct summary. They basically improve readability, so certain combinations of letters don't look smushed or interfere with each other--these are typically problems with 'fl,' 'ffl,' 'fi,' and 'ffi,' where l's are too close to f's, and dots above i's mush into overarching letters like f.
If you're going to allow the option, Obsidian, I would like to see them done correctly! It's not far off--just one thing has to be removed: stylistic or discretionary ligatures, which are for adornment only, and does not improve readability. In fact, many would consider them distractions, because they draw undue attention to themselves. In most settings, good typography is not heralded by fanfare, but should ideally go unnoticed; that means things are simply comfortable, easy to read, well-spaced, sized, etc. In typesetting software, ligatures are pretty much assumed if the font has the option, but discretionary ones are off by default.
There is very little reason anywhere to ever connect letters like 'st' or 'ct,' especially in any text outside of decorative headlines. I have for sure seen the st ligature in the game, but I'm not sure about the ct (it would appear in such words such as victory, tincture, actor, etc).
Again, I must acknowledge that in the face of many, far more pressing bugs, glitches, UI inconsistencies, etc, this detail is so, so minor. But hey, they've provided the option, so I consider it fair criticism!