FAR 61.183(i)(1) requires that a Flight Instructor candidate receive a logbook endorsement certifying proficiency in slow flight, stalls and spins.
How long has the requirement for spins been in the regulations?
Is it possible that there are CFIs in the field that do not have this endorsement?
I can't find a definitive answer but it looks likely - but not certain - that endorsements were introduced sometime between 1980 and 1991.
An AOPA article says that spins became an instructor-only thing in 1949, per CAR Amendment 20-3 (which I can't find online). That doesn't say anything about when an endorsement was introduced, though.
AC 61-67C - Stall and Spin Awareness Training has some historical information on page ii, including this (emphasis mine):
In January 1980, the Federal Aviation Administration (FAA) announced its policy of incorporating the use of certain distractions during the performance of flight test maneuvers. This policy came about as a result of Report No. FAA-RD-77-26, General Aviation Pilot Stall Awareness Study, which revealed that stall/spin related accidents accounted for approximately one-quarter of all fatal general aviation accidents. National Transportation Safety Board (NTSB) statistics indicate that most stall/spin accidents result when a pilot is distracted momentarily from the primary task of flying the aircraft. Changes to part 61, completed in 1991, included increased stall and spin awareness training for recreational, private, and commercial Pilot Certificate applicants. The training is intended to emphasize recognition of situations that could lead to an inadvertent stall and/or spin by using realistic distractions such as those suggested in Report No. FAA-RD-77-26 and incorporated into the performance of flight test maneuvers. Although the training is intended to emphasize stall and spin awareness and recovery techniques for all pilots, only flight instructor-airplane and flight instructor-glider candidates are required to demonstrate instructional proficiency in spin entry, spins, and spin recovery techniques as a requirement for certification. Part 61 was extensively updated in 1997.
In version A of the Flight Instructor PTS (May 1991), candidates are required to demonstrate a spin but the examiner can accept an endorsement instead, at their discretion (pp. 2-xliii - 2-xliv):
At the discretion of the examiner, a logbook record attesting applicant instructional competency in spin entries, spins, and spin recoveries may be accepted in lieu of this TASK.
The task itself is described as:
Demonstrates and simultaneously explains a spin (one turn) from an instructional standpoint.
So we can say for sure that since 1991 at least, instructor candidates have been required to either spin the aircraft during their checkride or have an endorsement saying they were capable of doing it. (I say "since 1991" because the same wording is still in the CFI PTS today.)
Of course, getting the endorsement requires spinning anyway (see AC 61-67 again), meaning that the candidate always had to spin an aircraft in order to qualify. The endorsement wasn't a way to avoid doing that.
I can't find any earlier sources or PTS versions that show the pre-1991 requirements, maybe someone else will have more luck.
And to answer your second question, 1991 was 'only' 29 years ago. It seems very possible that there are instructors who have been active for longer than that. If you're wondering if some instructors out there may never have spun an aircraft, the AOPA article implies that they've 'always' been required to spin aircraft anyway, regardless of exactly when the endorsement became an alternative to doing it in the flight test.
