Seems this topic comes up every other week, but I never get tired of it.
- Sentimental favorite, as always: "Flake off!" from Footloose, or maybe Dirty Dancing on TBS. Because it became a week-long meme when we saw it on our vacation.
- Something something stranger in Alps something something Mr. Falcon something Monday to Friday plane something something CGI bras in Showgirls something...
- The Fly, as seen on TBS or TNT, once cut out all of Brundle's final transformation. So you see humanoid Brundle one moment in the climax, then fly monster Brundle the next.
- Some stations got real lazy and just muted out curse words. Carrie on TBS/TNT, for instance. Yet for the longest time they neglected the "Carrie White eats shit!" on the school door.
- Grease 1 and 2...hoo boy... recalling the ABC airings in the 80s in particular...skipped scenes, skips in the dialogue and songs...in G2, you lose ALL of Reproduction so you literally see the students enter the classroom, then leave next second.
- Body Slam on daytime TV used a ring bell to edit out swear words. Yet one print kept the original crowd in the Ring Talk audience, one of whom had Billy Barty's favorite insult on a sign. (this changed to an alternate take with the sign saying "WIMP".)
- Santa's Slay...violence, cursing (muted), and the gay stereotype police officer somehow goes from being Toby Caulk to Dick Zucker.
- I remember a WPIX airing of Nightmare on Elm Street 4 that pixellated nudity.
If the OP meant "forgotten" edits....
- Not profanity, but Cockney Stiletto has been pretty much abandoned for the original Italian accented voice in Danger Mouse on streaming or physical.
- Also not profanity, but the 2000 prints of MTM's dramas (Hill Street Blues, Remington Steele, St. Elsewhere, and White Shadow) are now a bit rarer. These ones cut out the MTM logo in favor of 20th Century Fox's logo, but most reruns and official media keep the MTM ending now!
- Usually the cursing is all intact these days on A Christmas Story, but I remember older prints muted "son of a bitch" in the Ovaltine scene.
- Streaming, the "blonde loincloth" version of Splash on Disney + is gone.
- E.T. 20th Edition.
- The two part version of The Raccoons and the Lost Star seems to be the rare version now.
- Similarly I doubt we see much of the edited season 1 episodes from Disney Channel, nor the S2 ones that still use the Steve Lunt Run With Us.
- Are the Special Edition TOS episodes of Star Trek still out there, or have they been Spielberged?
On the other hand, if you meant ORIGINAL lines and scenes lost to time...
- Star Wars original trilogy. Just...STAR WARS ORIGINAL TRILOGY.
- The de-fired Beavis and Butt-heads now being "canon"
- Seems most Absolutely Fabulous releases these days are the ones with no Wheel's on Fire and other edits (Edina quoting Madonna, for instance)
- It's Your First Kiss, Charlie Brown, officially, will never use the original print where Peppermint Patty blames Charlie Brown for ruining the football game.
- If Fat Albert ever resurfaces and gets to Busted, they will usually use the one with toned down swearing.
- Good luck seeing the original end credits/closing logo to the series finale of St. Elsewhere on any official media now.
- If Game Show Network or others have not aired certain episodes of Fremantle-owned game shows when they kept "non-PC" words or clues intact, good luck ever seeing the uncensored version again, as Buzzr censors or skips episodes now. Doubly so for shows that have not been seen till now, like the Match Game/Hollywood Squares Hour.
- If your nostalgic station has updated/DVD quality prints of Paramount Television shows, don't expect to hear all the original music. It's highly unlikely you'll ever get the "pure" versions of certain WKRP episodes officially. Same for other examples; any DiC Nintendo cartoon for example.
- This may happen to The Raccoons and the Lost Star, as the streaming (in Canada) version edits out all the John Schneider songs and redubs his lines.
- Official releases of Chip 'n' Dale Rescue Rangers will all have the remade opening and end credits.