I'm a native American English speaker and I just used the conjunction "lest" twice last night in my five-page English paper for an American Literature class I'm taking, and I also said it today in class to two students whom I was talking to (about three times in 15 seconds as I was trying to get my point across).
Here are the quotations from my paper:
"This quotation is still relevant today because it epitomizes
self-reliance and individualism as a whole—the concept that we should
do our best not to fall into the trap of relying on others lest we
succumb to conformity, thereby losing our individuality."
"It is essentially a wake-up call to us that we must think for
ourselves lest we become like the sot in Emerson’s parable."
The situation today in class wherein I used "lest" is as follows:
"I made sure not to use the neuter 'he' in my paper again even
though Emerson uses it throughout 'Self-Reliance' because I know how
Sister Cynthia thinks it's sexist language. Since I'm attending a
college that is 93% female, I figure that I should be as pro-feminist
as possible lest I be chased off campus with pitchforks."
The two students responded jokingly that they don't have pitchforks in the dorms, so they won't chase me off campus with pitchforks, but maybe with something else. So based upon my use of "lest" and still remembering the day in second grade when we learned the word "lest" as our teacher wrote it on the chalkboard in a sentence, I think it is still a very common word albeit very formal. Yes, you can text "lest I forget"; it's a lot faster and simpler than typing out "in case I forget". We're always trying to save time in text-speak anyway, right?
NOTE: Remember that if you use the word "lest", it is followed by the present subjunctive conjugation of the verb in English.
I hope that might have helped you out. Take care and good luck.
P.S. Here is another excerpt from a paper that I just wrote last night. In my envoi, I state:
"The famous philosopher Baltasar Gracián once said, “All that really
belongs to us is time; even he who has nothing else has that.” It is
by this credo that Le Ly Hayslip has lived her life and it is by this
credo that we should live ours lest time itself pass us by into
oblivion like a wraith of smoke from the extinguished fire of our