I read the whole thing. I followed one or two of the embedded links.
What I found was a degree of misinterpretation (at best) of some of RMS's public statements, and quite possibly a failure to recognise RMS's own (it seems to me) "disability", namely: I think RMS may be quite autistic[1]. What gives me that idea is his focus on linguistic minutiae and strict definitions of terms.
I am for strict definition of terms in scientific and legal intercourse. In general conversation though words' meanings become more tentative and I suspect RMS may fail to recognise that.

For example his argument about the difference in definitions of "rape" between different cultures and nations is understandable. AIUI what might constitute rape is not the same in all States in the USA. Here in Britain there is no legal concept of "statutory rape" but British law has degrees of seriousness of the crime of having sexual intercourse with a minor: 13 - 16 year olds are minors but Home Office guidance on the law in their respect is clear that there is no intention to prosecute teenagers under the age of 16 where both mutually agree and where they are of a similar age.. Children of 12 or less years old cannot legally give their consent to any form of sexual activity. There is a maximum sentence of life imprisonment for rape, assault by penetration, and causing or inciting a child to engage in sexual activity.

Some of his comments suggest to me a lack of understanding of human behaviour and motivation. Like all of us now he is in a position publicly to express his opinions however ill-founded.

All in all, if the authors and signatories of that letter want to disassociate themselves from GNU and the FSF, they are at liberty to do so. Many of those who stay in the organisation may argue points like those with RMS, others may put them down to him being limited in human understanding, or a bit of a nutter or whatever.

I do wonder whether some of the stuff I've read about RMS recently hasn't been witch hunting - I am told that's a popular sport in the USA and has been since the time of those legendary Founding Fathers[2] :o)

[1] For clarity here: I do not think autism is a disability although it is characterised as such by many people and organisations. I have worked with/for people who were autistic in some way or other and the commonest problem they faced was that our culture and society are not constructed in such a way as to accommodate their needs.
[2] Witch-hunting is a popular sport in all the public/social media. Its activists seem to me to lack any understanding or forgiveness for human fallibility and utterly fail to acknowledge that they too are human and fallible.