Apparently this is now more common. IF you do submit a LOCI, make sure it is a school you will accept an offer from, and also make sure the school accepts them:
Some schools may not even accept a letter of continued interest at all. University of Virginia for example, notes that letters of continued interested are not used as a factor in their admission process and ask that you not send letters or updates outside of a midyear grade report. And USC recently posted on Instagram reminding students not to submit Letters of Continued Interest. If you see that your prospective school does not accept these types of letters, be sure to abide by their request.
Dartmout accepts LOCI’s.
Write a Letter of Continued Interest swiftly, which you should email to your regional representative on the admissions staff and upload to the portal.