My point is that you are then deigning someone to have the power to decide what is tolerant and what is not, when that is not nearly as acceptable or uncontroversial as you seem to be assuming. For example:
> If progressives are pushing for more freedom, more tolerance, then to oppose it you have to, by definition, advocate for less of those.
The framing of this is not correct. The problem is that not all "progressives" are pushing for that, which then undermines the assumption that opposition is therefor automatically intolerant and therefor automatically bannable/deplatformable. Some of the real progressives I follow such as Graham Elwood and Jimmy Dore have frequently had issues with their platforms giving them a hard time (demonteizing, etc) just because their positions or even topics of conversation are controversial, and many of them frequently mention they wouldn't be surprised to be deplatformed for said reason.
To me, the real crux of your position, is that by completely bucking the standard of free speech in which we have legal definitions for speech that isn't tolerable, positions like yours seem to want to expand those definitions (libel, defamation, imminent incitement to violence, etc) to a point that you must now designate some non-legal entitiy as the moral arbiter of what is intolerant/tolerable, and that is indeed a most slippery slope.
I think I have rewatched these two videos of Christopher Hitchens on the topic a good half dozen times each in the last year, and I keep thinking about their content. [1,2] From [2], I will link the timestamp of what I am about to quote [3]: "Whose going to decide? ... Who will you appoint? Who will be the one who says, "I know exactly where the limit should be, I know how far you can go, and I know when you've gone too far, and I'll decide that" Who do you think, Who do you know, who have you heard of, who have you read about in history who you would give that job?... Not for you, but for the people you are listening to and the peoples whose comments you hope to hear in return, for your own education, for your own elightenment, and for your own elucidation, as Mr. Paine says, commenting on Milton, one of the vices of those who would repress the opinions of others is they make themselves prisoners of their own opinions, because they deprive themselves of the means of changing them."
1. https://www.youtube.com/watch?v=4Z2uzEM0ugY
2. https://www.youtube.com/watch?v=Tjhj9Hn8oQw
3. https://youtu.be/Tjhj9Hn8oQw?t=2590