So firstly the mandatory disclaimer: free speech is about freedom from government restrictions on speech, Twitter or SDMB, or any other private entity deciding they don’t want to publish your speech is not a restriction on free speech (in fact it’s Twitter, SDMB, practicing their free speech right to decide what speech they promote)
But in terms of actual free speech restrictions and how it’s a bad idea look no further than the UK. The lack of a well defined concise freedom of speech under the British constitution has meant outright cut and dried infractions on freedom of speech by the government are now common place. I could cite numerous examples but this one appeared on my social media feed this morning. The Home Secretary has stated she believes: “'From the river to the sea, Palestine will be free’ should be understood as an expression of a violent desire to see Israel erased from the world”, and should be prosecuted under hate speech laws.
“It is not just explicit pro-Hamas symbols and chants that are cause for concern. I would encourage police to consider whether chants such as: ‘From the river to the sea, Palestine will be free’ should be understood as an expression of a violent desire to see Israel erased from the world, and whether its use in certain contexts may amount to a racially aggravated section 5 public order offense.
That is about as blatant infraction of freedom of speech as you can get, “From the river to the sea, Palestine will be free” is about as protected as protected speech can be, and does not come close being a threat or anything else that could (or should) be a justification for prosecuting it.
This is not some right wing blow hard running their mouth about some massively unconstitutional thing they think should happen but will never come close to becoming law (of which the US has plenty), this is the home secretary instructing police to prosecute people for this (not to mention it would not be a particularly radical departure from the types of things that have been prosecuted or arrested for in the UK in recent years, e.g: this or this).