I ask myself this question because of how the homebrew game City Trouble was developped, having on purpose sprites that looks very old-fashioned without sprite outlines. I wonder when the appeared.
The earliest games often had completely black backgrops, as the hardwired 8k of VROM did not allow for fancy backdrops. Sprites were made of simple colored surfaces, and this looked good enough (against empty black backdrops). Then games started to have either bankswitchable CHR-ROM or CHR-RAM, and were able to have more sofisticated graphics with backdrops. Games such as Zeldas or Super Mario Bros. have sofisticated backdrops but no outlines. This looks weird, so black outlines were developed as a response, to have sprites good looking.
When did that happen ? Apparently this spread from the end of 1986 very quickly but I'm not so sure.
Also some games have outlines with another colour than black, such as Gimmick having just "dark outlines" instead of black. This looks better against both a black backdrop and a coloured backdrops, but requires sprites to be very bright coloured (at least in the $1x palette range).
The earliest games often had completely black backgrops, as the hardwired 8k of VROM did not allow for fancy backdrops. Sprites were made of simple colored surfaces, and this looked good enough (against empty black backdrops). Then games started to have either bankswitchable CHR-ROM or CHR-RAM, and were able to have more sofisticated graphics with backdrops. Games such as Zeldas or Super Mario Bros. have sofisticated backdrops but no outlines. This looks weird, so black outlines were developed as a response, to have sprites good looking.
When did that happen ? Apparently this spread from the end of 1986 very quickly but I'm not so sure.
Also some games have outlines with another colour than black, such as Gimmick having just "dark outlines" instead of black. This looks better against both a black backdrop and a coloured backdrops, but requires sprites to be very bright coloured (at least in the $1x palette range).