Here is the full list of all 34 Sekiro: Shadows Die Twice achievements. It takes around 60-80 hours to unlock all of the achievements on Windows.
Defeated "Great Shinobi - Owl"
Grasped the inner mysteries of any combat style
Acquired all Prosthetic Tools
Acquired all Ninjutsu Techniques
Used the Mortal Blade to sever the Guardian Ape's undying
Defeated the "Great Serpent".
Defeated the "Guardian Ape"
Caught the Folding Screen Monkeys
Defeated "Genichiro Ashina"
Defeated "Lady Butterfly"
Defeated "Gyoubu Masataka Oniwa"
Encountered the Memorial Mob
Returned from the dead using "Resurrection" for the first time
Acquired the Shinobi Prosthetic
Received the "Kusabimaru" from Kuro
All achievements have been unlocked.
Defeated all bosses
Traveled to all areas of the game
Upgraded all Prosthetic Tools to their limit
Acquired all skills
Upgraded Vitality and Posture to their limit
Fully upgraded the "Healing Gourd"
Attained the "Immortal Severance" ending
Attained the "Purification" ending
Attained the "Return" ending
Attained the "Shura" ending
Defeated "Sword Saint Isshin Ashina"
Used Lapis Lazuli to upgrade any tool to its limit
Defeated "Great Shinobi - Owl" at the Hirata Estate
Defeated the "Corrupted Monk"
Defeated the "Divine Dragon" and obtained the "Divine Dragon's Tears"
Defeated "Isshin Ashina"
Defeated the "Demon of Hatred"
Defeated the "Great Colored Carp".