It's not unusual for Rockstar Games to leave content on the cutting room floor in their games. Red Dead Redemption 2 is not an exception. But, this doesn't make discovering cut content any less of a curiosity. Case in point, this potential quest line would have involved Arthur traveling to the restricted areas of the game, specifically, Blackwater and New Austin.

According to Kush Qrox, there's evidence in the in-game files of the PC version of Red Dead Redemption 2 suggesting that the quest involving Jeremy Gill originally went differently.
Apparently, there's an unfinished cutscene where Jeremy Gill, the angler who tasks players to catch all the legendary fish in Red Dead Redemption 2, effectively escorts players to Rio Bravo, which is a place in New Austin. In the final version, players can't complete Gill's questline until the epilogue. This is because Arthur is a wanted man in Blackwater and New Austin and will be gunned down by an unlimited number of bounty hunters if he steps foot in the aforementioned regions. Since the cutscene leaves Arthur somewhere in New Austin, it appears that Rockstar originally had different plans for Arthur in Red Dead Redemption 2.
GTA BOOM now has its own community discussion forum! Chat directly with our guide creators, ask questions, & get tips and advice.
Of course, since this is an unfinished questline, it's quite a stretch to think that Arthur could have stepped foot in New Austin. Don't forget, Arthur is a wanted man in Blackwater and New Austin right from the start. If we assume that this was somehow possible in the early stages of Red Dead Redemption 2, this would imply that Rockstar had different plans for the game's narrative. Although we aren't ruling this out, Red Dead Redemption 2 wouldn't have worked as well if Arthur wasn't an outlaw in those parts.
Ultimately, we defer to Rockstar's judgment for why it cut this specific piece of content from Red Dead Redemption 2. At the end of the day, we still got the fishing quest even though it's John who will finish it and not Arthur.
Speaking of John Marston, the voice behind the lead of the first Red Dead Redemption game just voiced his support for a potential remake or remaster. This comes after rumors of Rockstar dropping its original plans of re-releasing the 2010 title to focus on GTA 6.