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 the "Corrupted Monk"
Traveled to all areas of the game
Defeated "Great Shinobi - Owl"
Acquired all Ninjutsu Techniques
Used the Mortal Blade to sever the Guardian Ape's undying
Defeated the "Great Serpent".
Caught the Folding Screen Monkeys
Defeated the "Guardian Ape"
Defeated "Genichiro Ashina"
Grasped the inner mysteries of any combat style
Defeated "Gyoubu Masataka Oniwa"
Defeated "Lady Butterfly"
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
Upgraded all Prosthetic Tools to their limit
Acquired all skills
Acquired all Prosthetic Tools
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 "Divine Dragon" and obtained the "Divine Dragon's Tears"
Defeated "Isshin Ashina"
Defeated the "Demon of Hatred"
Defeated the "Great Colored Carp".