Missable Content in Redemption II

Unlike it's predecessor, Red Dead Redemption 2 contains missions, items and other content that can be permanently missed in a save file unless obtained, studied or otherwise interacted with during a certain time-frame. While anything on this list is not required for 100% Completion, a player may be interested in completing and collecting everything available in the game. As there is no official list of everything that's missable, this list may be missing some content or certain content may be available even outside these time-frames.

Guarma animals
All macaws spawn near the waterfall near the camp, the booby often flies above the river, and snakes spawn in the vegetation down the hill from the camp. Study and skin/pluck all animals to fill the compendium.

Birds

 * Scarlet Macaw
 * Blue and yellow Macaw
 * Great green Macaw
 * Red-footed Booby

Snakes

 * Fer-de-Lance snake
 * Red-tailed boa snake
 * Rainbow boa snake
 * Sunglow boa snake

Missions & Mission strands
Each mission/mission strand expires after the chapter is over, unless otherwise specified.

Chapter II

 * Money Lending and Other Sins I & II
 * We Loved Once and True I
 * We Loved Once and True II
 * We Loved Once and True III
 * Pouring Forth Oil II
 * Good, Honest, Snake Oil (Doesn't expire until epilogue)

Chapter III

 * Money Lending and Other Sins IV
 * The Course of True Love I
 * The Course of True Love II
 * The Course of True Love III

Chapter IV

 * Money Lending and Other Sins V
 * Fatherhood and Other Dreams I - II

Chapter VI

 * Money Lending and Other Sins VI & VII
 * Archeology for Beginners
 * Honor, Amongst Thieves
 * Mrs. Sadie Adler, Widow I - II

Chapter II

 * Fundraiser (Unavailable in the epilogue)

Chapter IV

 * Help a Brother Out
 * Brothers and Sisters, One and All
 * The Mercies of Knowledge

Chapter V

 * Willard's Rest encounters:
 * All encounters can only be seen by visiting Charlotte Balfour 3 times with Arthur Morgan, and then visiting her once as John Marston in the epilogue
 * If the player never meets Charlotte as Arthur, only the first 2 encounters will be available in the epilogue.
 * If the player meets Charlotte as Arthur but doesn't complete all encounters, she cannot be encountered in the epilogue as she has passed away before 1907.

Chapter VI

 * Of Men and Angels I - II
 * Do Not Seek Absolution I
 * Do Not Seek Absolution II
 * The Course of True Love IV
 * The Course of True Love V

Camp companion activities
Activities appear at camp randomly, and become unavailable at the end of each chapter once the gang relocates the camp.

Chapter II

 * Five Finger Fillet with Lenny Summers
 * Home Robbery with Javier Escuella
 * Hunting with Charles Smith
 * Dominoes with random gang member
 * Poker with random gang member

Chapter III

 * Coach Robbery with Sean MacGuire
 * Coach Robbery with Bill Williamson
 * Fishing with Javier Escuella
 * Home Robbery with Javier Escuella
 * Fishing with Kieran Duffy
 * Five Finger Fillet with Micah Bell
 * Dominoes with random gang member

Chapter IV
While not a activity in that it's not started on the camp, Javier Escuella can randomly ride to the player roaming the wilderness and request help rescuing Bill Williamson from bounty hunters. This will only happen before chapter V.
 * Bank Robbery with Charles (Special/Ultimate edition content)
 * Coach Robbery with Micah
 * Coach Robbery with Lenny
 * Hunting with Pearson
 * Rustling with Uncle

Camp item requests
Most requests trigger randomly between 8 am - 8 pm, but exceptions exist.

Source

 * https://www.reddit.com/r/reddeadredemption/comments/byj6ha/red_dead_redemption_2_missable_content_list/
 * https://docs.google.com/spreadsheets/d/1xFig-2M6HIRmhvcXlOl4K2zHMeGxHb71dku2dWOGwRc/edit#gid=837263948