many thanks as always brucey, but cripes - haven't seen anything that daunting looking since I escaped sunday school aged 10.
I may be some time.
That page is rather outdated.
* up to 9-speed largely the same, and Shimano acknowledge that you can mix and match derailleurs in their docs.
* 10 speed road (Dura-Ace 7800) - 2003 - still the same, but as more cogs are packed into the same amount of space, the amount of cable pulled to move one gear has got increasingly small, which makes the system fussy about adjustment
* 2008 - Shimano decides to hide its cables under the bar tape, as with Campagnolo and SRAM, for Dura-Ace 7900 (also Ultegra 6700 and 105 5700), which adds extra bends and friction. They revise the front derailleur pull ratio, and it works well. They do not revise the rear derailleur, and it works.... not so well.
* 10-speed MTB 2010 (Dyna-Sys) - several years have elapsed since 10-speed road, and Shimano decided that they would be better off pulling more cable (more shift force required, but easier for adjustment), as perhaps the old pull ratio was no longer well-suited to 10-speed. They pull much more cable than older generations.
* 11-speed road 2012 - Shimano decides to come up with a new ratio somewhere between the 10-speed MTB and the older ratio, so that there's not too much force required, but so that it can overcome the extra friction introduced by the hidden cables, so they now have '11-speed road' RDs (even though there is no 'speed' inherent to the RD, since the indexing is in the shifter) distinct from everything else.
* 11-speed MTB 2014 - Shimano realises it can make more money by claiming that everything is incompatible, so says you need a '11-speed MTB RD' for your 11-speed shifters, even if 10-speed worked fine, people are encouraged to 'upgrade' (which might have been necessary, as 11-speed came with larger cassettes, though you could in fact use a 11-speed HG700/HG800 11-34t cassette just fine)
* 2015 - Shimano wants to update their 10-speed Tiagra groupset to have hidden cables, which the previous generation did not, in order to provide a distinction between what-was-then 10-speed 105 and 10-speed Tiagra; now 105 is 11-speed, so that's enough distinction to keep the marketing people happy. Rather than do what they would previously do, which is churn out a re-painted old 105 10-speed derailleur/shifter, they acknowledge that it was a bit rubbish, and instead they modify their 11-speed 105 shifters for 10 speeds, and stick a 'Tiagra' sticker on the '11-speed' RD
* 12-speed MTB 2018 - more of the same as with 11-speed: Shimano claims you need a specific 12-speed MTB RD
* 2019 - Shimano releases a new 'gravel' (aka 'road') product line, as that's the latest way to print money. To create arbitrary product differentiation, they have a '400' series at 10 speeds, '600' & '810' at 11-speeds. They confirm that they've dumped the old pull ratio, as both use the same derailleurs.
* 2020 - Shimano updates their MTB groupsets to have more cogs and poorer build quality. They release two '11/10' speed rear derailleurs, confirming there was never any difference between '11-speed MTB RD' and '10-speed MTB RD', whatever they said in their charts.
The 11-speed MTB generation ('DynaSys 11') was Shimano's last attempt to promote multiple chainrings to the hostile market, and they put out several graphs showing how crappy 1x is:
It's based on a 11-40t cassette with a nice graph showing how well-spaced it all is;
The market was not impressed, so Shimano had to put out a 11-42t cassette which replaced 24-27-31-35-40 with 24-28-32-37-42.
This wasn't good enough either, so they pushed out a 11-46t which simply replaced the 42t with a 46t, which was awful rubbish, but sold like hotcakes as a '1x system'.
The 12-speed generation is sadly absent any graphs showing how much less efficient it is than the preceding ones (though to be fair, they have put out 36/26 doubles for 12-speed, it's just they don't sell, because everyone wants massive heavy weights on their rear axle), but at least Shimano knew they had to design it around 1x, so the boat anchor cassette was there from day one.
Shimano's original 11-speed MTB with a 3x chainset and a 11-40t was a very good design, but a sales failure and saw them destroyed by SRAM which boasted bigger, more expensive cassettes, RDs with more plastic in, and other such delights, so Shimano learned its lessons and have dutifully downgraded the quality and upgraded the marketing, rebranding 'Alivio' as 'Deore' and other such tricks, and they seem to be re-taking marketshare.
One presumes the delay to 12-speed road is the conflict between the desires of the marketing department and those of the engineers.