In some ways yes, but it already happens. The way reactionaries frame the debate (e.g. freedom of expression is the freedom to harass minorities with slurs on campus) is actually a fairly recent project starting in the 90s (perhaps to discredit freedom of expression as a goal). The leftist approach to freedom of expression (e.g. advocating for the people our state is actively bombing, private/personal property, whistleblower attacks, intellectual property etc.) tends to be a lot more nuanced.

I think the idea of the slippery slope is a bit banal. It's not really swaying people one way or another, it's just people will find it as a convenient excuse to "both sides". e.g. Australia banning both Nazi salutes and Hamas flags. This didn't need prior justification to do so, inasmuch as Australia has free speech laws to begin with. If supposedly "free" capitalist states can do such things, why not a socialist state? It makes them only just as bad on that one issue, not worse, which kinda means that if you feel such worry about a future socialist state, you should feel the same worry about current capitalist ones.

To say nothing of how any restriction would be punished. Imprisonment is worse than a fine is worse than rejection from a campus is worse than rejection from a social group. But all societies practice a grading scale of all of those (wrt. speech).

I don't think you're crazy for worrying or even thinking about it. I think it will come down to the circumstances of whatever revolution happens and what groups are involved in negotiating the future. Personally, I'd want to promote active debate both inside and outside the party, but your right to speech is limited from recruiting foreign actors to help overthrow the government and conspiracy to commit crime (depends on the crime). Maybe something about active military movements (should such a thing exist) during a conflict? Even if you're a one party state, you want to have a lot of ears on the ground to address particular grievances, however mundane, and safe discussion is a good place to explore ideas. (this is something that I disagree with right wingers on, if a lot of minorities feel unsafe even if the state isn't coming down on you, you won't get productive discussion, you'll get a circle jerk of the dominant ideology)