How has the frequency of FIDE rating lists changed over the years? Today FIDE publishes a new rating list once a month. When FIDE first started publishing official lists in 1971 they came out once a year. How has the frequency changed over the years? When did the frequency change and by how much?
The period before 2001 is documented on the Olimpbase site. The period from 2001 on is documented on the FIDE rating site.

To find the details for pre-2001 go to the Olimpbase site and about a third of the way down on the right hand side under "PLAYERS &amp; TEAMS" click on "Elo lists 1971-2001".

To find the details for 2001 and after go to the FIDE rating download site and halfway down click on the dropdown box "Download old files from Archive - Please select your period and press GO".

Both these sites give the possibility to download the lists in zip file format.

Pre-2001

Unofficial lists were published in -
June 1967, April 1968, 1969 (no month given), 1970 (no month given) and January 1971

Official lists were published in July of the following years:
1971, 1972, 1973.

This switched as a one-off to May in 1974 before settling down to full lists published in January in 1975 through to 1980 with the addition of three supplementary lists in July of 1978, 1979 and 1980. These supplementary lists included less than 10% of players.

In 1981 FIDE switched to publishing lists twice a year, in January and July, and this continued from 1981 through to 1999.

This period was also significant for the introduction of a number of new features:


FIDE IDs in 1990, although the first attempt in January 1990 only had
5 digit IDs. This was clearly going to be inadequate and the length
was extended in July 1990.
With the break up of the USSR new federation codes were introduced
for ex Soviet, Yugoslav and Czechoslovak republics.
"Active / Inactive" flags were introduced in July 1992.
The rating floor for men was lowered to 2005 in January 1993.
Ratings stopped being rounded to the nearest 0 or 5 in January 1999.


In 2000 three rating lists were published in January, July and October.

In 2001 the frequency changed to 4 times a year. Lists were published in January, April, July and October from January 2001 up to July 2009.

From September 2009 through to July 2012 lists were published every two months in January, March, May, July, September and November.

In January 2013 they changed to being published monthly through to the current day.   

Although it should be noted that the actual download files themselves are updated on an almost daily basis. For instance, I am posting this on 9th September 2019 and that is also the date on the download file. Two days ago the date was 7th September. The information in the two files is almost the same. The differences are minor corrections and additions. The data in the September files is still significantly different from the August files in that the August files reflect data reported in July while the September files reflect data reported in August.