Freedom of speech
Freedom of speech is a fundamental human right which means that an individual has the right to express thoughts and ideas without any fear of legal action, retaliation or censorship. It is a requirement for a free society and it is also a requirement for a functional democracy.
A fundamental human right
Freedom of speech is recognized under article 19 of the Universal Declaration of Human Rights (UDHR). Countries with governments who frequently use legal action or other violent action to silence political speech, such as Norway, Sweden and other European countries as well as some Asian countries like North Korea, are in violation of this article. They are also not truly democratic countries since freedom of speech is a requirement for a functional democracy.
Relation to free software
Code is a form of expression. Free speech is therefore a fundamental requirement for a thriving free software community. Respecting freedom of speech is not a requirement to call a piece of software "free software", Free Software which Censors and Restricts what Sites and services the user is Allowed To Read and use is still Free Software. Free speech is a requirement to distribute free software freely and it is also required to have meaningful discussions around the development of free software.
Censorship of "harmful opinions", "hate speech", "hateful" or "harmful" speech
What constitutes "hate speech" speech is in the eye of the beholder, it is completely subjective. Embrasing rules which forbid what someone could consider "harmful" or "hate" is the same as abandoning freedom of speech as a concept. Speech you personally hate reading or seeing may be perfectly acceptable to others. Someone could strongly hate a joke because they don't get it. Someone may hate seeing logical and rational arguments they can't argue against.
Embracing the fundamental concept of free speech means accepting speech you disagree with. This does not mean that you should go out of your way to hurt others or be harmful with your speech, acting civil is always good thing to do. It simply means that freedom of speech and freedom of expression should be protected. That freedom of speech can be used to explain to people who are hurtful that they should not be acting the way they are acting - which differs from outlawing and forbidding the speech.
"I want free speech, but.."
There's no "but..". The concept of freedom of speech is gone the moment you say "free speech but not z, x or z. There is no free speech when an authoritarian state, private multinational corporations or other powerful groups dictate what is and what is allowed based on what in most cases are vague, unclear and rapidly shifting rules or guidelines.
Exceptions to the fundamental concept of freedom of speech is specially problematic on the Internet where content tends to remain available for a very long time. Something perfectly legal today could become illegal 10 years later and lead to prosecution or other consequences if freedom of speech is not defended.
Examples Of Platforms With Shifting Rules
YouTube changed their guidelines for what their platform allows in December 2019. The new rather vague "rules" lead to an immediate purge of many videos and whole channels which had existed on that platform for years and years.
The "First they came ..." poem
First they came for the socialists, and I did not speak out—
Because I was not a socialist.
Then they came for the trade unionists, and I did not speak out—
Because I was not a trade unionist.
Then they came for the Jews, and I did not speak out—
Because I was not a Jew.
Then they came for me—and there was no one left to speak for me.