Channel

Standard	Rotated out on April 23, 1997
Pioneer	Not Legal
Modern	Not Legal
Legacy	Banned since May 1, 1996
Vintage	Restricted since October 1, 2000
Commander	Banned since June 20, 2010
Pauper	Never Banned
Oathbreaker	Banned since June 19, 2018
Extended	Rotated out on October 1, 1999

View on Scryfall

Timeline

• August 5, 1993 - Released
• March 23, 1994 - Status changed to Restricted in Vintage
• January 10, 1995 - Status changed to Restricted in Standard
• November 1, 1995 - Status changed to Banned in Vintage
• November 1, 1995 - Status changed to Banned in Standard
• May 1, 1996 - Status changed to Banned in Legacy
• April 23, 1997 - Status changed to Rotated in Standard
• July 1, 1997 - Status changed to Banned in Extended
• October 1, 1999 - Status changed to Rotated in Extended
• October 1, 2000 - Status changed to Restricted in Vintage
• June 20, 2010 - Status changed to Banned in Commander
• June 19, 2018 - Status changed to Banned in Oathbreaker

Rationale

While we don't have official banning statements from WotC during the period this card was banned, Channel was notoriously a part of the game's first gamebreaking combo alongside Fireball, and it's a fair assumption that that was the catalyst of its limitation in early formats. It's ability to produce absurd amounts of mana very quickly also eventually got it banned in Commander and Oathbreaker.