UPDATE
I will no longer be updating this haphazard record (haven't been for almost a year, in fact).
Also, the GOG community wiki that contained the page I linked to throughout this post is down, probably for good. An archived version is here. (I will leave the original links unchanged for "historical purposes".)
END UPDATE

I got weirdly obsessed last night (after seeing the "Pony Island removed?" thread) with looking once more into the details of the removal, occasional return, and even more occasional re-removal of games, chronicled here -- so much so that I made a document in WordPad breaking the numbers down by year, with some additional qualifications. So I figured I might as well post it here, too. :P

This does not count games that were replaced by or bundled in with an expanded/improved version the same day the removal happened (or before). It also doesn't count demos and such. Additionally, I don't count different "content editions" of the same game as separate; for example, if a game that released with basic, Deluxe and Legendary editions (which differ mostly or entirely in the amount of out-of-game "goodies" included) -- and which also has DLC available whose sole purpose is to upgrade from a lower-tier edition to a higher-tier one -- gets all its editions and associated DLC pulled from the store, I would only count it as one game removed (nor would I likely count the upgrade DLC at all in such a case). I will try to count any DLC adding content not available in any edition of the base game, however.
Most info taken from the GOG Wiki page, and from (some of) the relevant pages linked to in that page's references. If something's off, I suppose let me know...though I cant say for how long I'll bother keeping the numbers, etc., up to date. Also, be aware that any errors on the wiki are not mine. :)
Each of the parenthetical notes should be read as commenting on the initial figure of the "paragraph" ("[#] games removed" or whatever), NOT as commenting on a preceding, separate parenthetical note. I might rework this to be more intuitive at some point, as it bugs me, but again, this was originally written for my own amusement, so I wasn't too worried about it being intelligible to anyone else, as long as it was internally consistent. :)

2009:
4 games removed (of which 1 came back just a few months later, another 1 eventually, though under a different name, & w/ less content);
1 re-added

2010:
2 games removed (of which 1 came back eventually)

2011:
2 games removed (of which 1 came back just a few months later);
2 games re-added

2012:
6 games removed (of which 2 came back after just a month and a half);
3 games re-added

2013:
9 games removed (incl. 1 whose imminent removal was announced in late Dec. 2012, and which was removed early the 1st of Jan.) (of which 2 came back within 5 months);
2 re-added

2014:
46 games removed (of which 4 had been previously removed and re-added, 1 of these in slightly different form) (though 1 was only unavailable for a couple days before being replaced by its special edition; of the remaining 45, all but 7 came back [though 2 were later removed again], incl. 2 after just 2 months);
3 re-added (+ 32 more which were made temporarily available for a weekend promo)

2015:
4 games removed (of which 3 eventually returned, though one of these was split in two for its re-release);
38 games re-added (of which 5 first returned temporarily again for an Insomnia sale, before coming back permanently a couple months later)

2016:
10 games removed (though 2 were versions of the same game) (incl. 4 whose imminent removal was announced in late Dec. 2015, and which were removed on the 1st of Jan.) (of which 1 came back just a few months later);
1 game re-added

2017:
29 games removed* (of which 2 had been previously removed & re-added) (of which 2 came back within a few days, 2 more within a month, and 3 more later the following year, one of those under a different name) (incl. 1 In Dev title which was pulled from sale, but remained in the catalog as an upcoming title until being replaced by the differently-named release version over a year later) (incl. 1 whose imminent removal was announced in late Dec. 2016, and which was removed on the 2nd of Jan.);
at least 7 DLC removed (of which 4 were out-of-game-content), all for removed games;
1 movie removed (TPB AFK: The Pirate Bay Away from Keyboard);
8 games re-added (of which 2 were previously sold as one package)

*Not counting Sorcerer King, but counting Descent: Underground

2018 [WIKI LIST NOT UP TO DATE!]:
25 games removed* (of which 2 had been previously removed & re-added) (1 of which was an older version of another game that's still available) (incl. 1 In Dev title which was pulled from sale, but which remained in the catalog as an upcoming title, and another that was pulled from the store just as it came out of In Dev, as well as 1 unfinished episodic game which was pulled from sale but left in the catalog as an upcoming title, only to later be removed entirely) (1 of which came back several months later, once it reached "full release" status);
2 DLC removed (each for a different also-removed game);
at least 8 movies removed**;
5 games re-added† (2 of which were removed again a few months later)

*Also counting Startup Company, Star Control: The Ur-Quan Masters, Star Control: The Kessari Quadrant, Rollercoaster Tycoon 3: Platinum!, Human Fall Flat, Everything, RealMyst, Gray Matter, Fez, MIND: Path to Thalamus Enhanced Edition, TerraTech, The Walking Dead: The Final Season, Perception, The Long Dark, Industry Giant 2, Lula: The Sexy Empire, Republique, The Walking Dead: Season 1, The Walking Dead: Season 2, The Walking Dead: Michonne - A Telltale Miniseries, The Walking Dead: A New Frontier and Wargame: European Escalation (all of which someone with GOG Wiki access should add to that list!), as well as We Happy Few; not counting Softporn Adventure (which was removed from the Leisure Suit Larry [- Greatest Hits and Misses] package, the rest of which remained available)

**Counting Dark Dungeons, The Gamers: Director’s Cut, The Gamers: Dorkness Rising, The Gamers: Hands of Fate, The Gamers: Humans & Households, The Gamers: Natural One, JourneyQuest and JourneyQuest: City of the Dead.

†Also counting We Happy Few, Call of Cthulhu: Dark Corners of the Earth and Alan Wake