For this thread just go with shows on the big 4 (ABC, CBS, FOX, and NBC) networks, not netlets like CW or anything cable and streaming. Last night I was surprised to see Jason Alexander guest star on The Nanny, what was their season 4 premiere on CBS. This was in 1996 when Seinfeld was red hot starting its 8th season, you wouldn't think NBC would have allowed this.

I know in the 1970s, Charlotte Rae had a contract with CBS which prevented her from doing a show (Diff'rent Strokes) on NBC, but Norman Lear was able to get her out of that contract.

I think by the 1990s, the exclusivity clauses had mostly become a thing of the past. I don't think the networks were able to stop actors from appearing "on another network" anymore. For example, I know Norm MacDonald, while contracted to NBC for Saturday Night Live, still made guest appearances on The Drew Carey Show (ABC).