If that yellow is not soft it is probably not rot, just that it is older than the younger pups and a darker color. This is the green (not variegated) tall form of S. trifasciata? They are pretty forgiving, within reason. I personally would not remove all the soil on those roots unless they were obviously rotting (soft, mushy, smell bad). Sit the root ball on a paper towel to help draw off extra moisture and then repot with good soil below and above that current root ball.

Hello Lena,
Thank you for the advice. I'm responding late because I was asleep earlier and spent all morning outside. I have some perlite for the potting mix now and just took a picture of the large leaf. It looks a bit less yellow on the inside, but there is that brown spot I didn't notice before.
Are you sure I should leave all the roots out to dry? I was wondering if it'd take the plant longer to establish itself after repotting if I did that, since I don't know if the smaller leaves still have healthy roots or not and if I'd be killing them off for no reason if I did that.
It's probably worse to leave it in moist soil and let the rot(?) progress further though
I'll take the leaves out and start drying if no one else is around to respond, since I know some sites recommend you let the end of a leaf snapped off the mother plant dry to form a callus when you're propagating

Hello woodnative,
It's the variegated form. The older leaves get yellow around the edges, but I didn't know the colour of the base would change too. Don't really want to pull up any healthy leaves to check right now.
I checked every part of the leaf around the yellow part of the base and none of it was soft at all. That makes me feel a bit more hopeful, thanks.

Pulling it up to check roots for rot does no harm WHATSOEVER. Far more harmful is to be so wet as to rot. Pls. note that while WoodNative & I may disagree on changing the mix, we clearly agree to pull the whole thing up out of the mix & let it dry on some absorbent paper. That is key to trying to prevent rot.

Was going to suggest after drying for a day, sprinkle some cinnamon (like you use in baking) on the lower leaf that's yellowed & shake it off before repotting. It acts as a natural anti-fungal (anti-rot) which I learned from orchid growers on their Forum.

Now I see that yellowing leaf looks like 1 single leaf. If true & it were mine, I'd remove the whole leaf from the bottom. Good luck.

Okay, I've pulled out all the leaves and the roots are drying on paper.
I smelled the roots again and I think I was just mistaken when I checked yesterday because it was late, the roots just smell like fresh soil, it's almost a pleasant smell.
Maybe there's no point, but I know nothing's going to die this way so thank you both.

Hello again Bora,

Good you pulled it out to dry (for a day or 2), but just to be clear, I never said

"... nothing's going to die this way ... "

What I said was: "These are pretty rugged plants, could be there's hope".

Hopefully, you can get some fresh NEW mix, am suggesting Cactus & Succulent mix & a bag of perlite & mix 50/50% & repot DRY. Then pls. don't water for a few days. If you feel the need to settle the plant, just bang the newly potted plant on or against your table a time or 2. That will settle the new mix w/out watering.

Fingers crossed for ya & Merry Christmas!

Good advise given. Snake plants can live without water for months. But in days it can die from rot. Best to pull out and let dry and pot up with new soil w/perlite and do not water. Start watering a week or two later and only just a tiny bit.

How is your plant doing now?

Sorry for the late update, but I pulled it out to dry, repot the leaves separately and they've grown new shoots since.
The other 2/4 of my snake plant pots have also flowered since then.
Thanks for all the help!