Does anyone know when/if likely letters get sent out? Whether Early Decision applicants who got deferred can get them? Whether receiving a likely letter guarantees admission?
For the RD round, they generally go out around March 1 +/- a couple of days. If you get one, it’s the same as getting an acceptance letter, subject to the same terms (don’t get arrested, don’t fail courses, etc.)
Keep in mind, likely letters are like fairy dust. The vast majority of accepted students do not get a likely letter. In fact, only about 150 are sent to non-athletic recruits. So don’t keep refreshing your email in the hopes that one will show up. 
@khuie2017, likely letters definitely won’t be sent to a student that applied ED but was deferred. The LL is used by the Ivies to let an especially desirable RD applicant know in advance that the student will be admitted. Brown would not send such a letter to an ED applicant; instead they would just accept that applicant ED. As I understand it, the other use of the LL is to give recruited athletes who have applied RD (after previously had a pre-read by admissions) the confidence to flip their application to ED.
They definitely also send LLs to recruited athletes for the RD round. March 1st allows football recruits who might be deciding between ivies and non-ivies one month before the end of their signing period to mull it over knowing they have an acceptance to an ivy in hand. All the other sports are later (http://www.nationalletter.org/signingDates/)