What is the history of the CFI spin endorsement requirement?
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 &quot;since 1991&quot; 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.