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.
Attained the "Purification" ending
Upgraded all Prosthetic Tools to their limit
Defeated "Great Shinobi - Owl" at the Hirata Estate
Upgraded Vitality and Posture to their limit
Attained the "Return" ending
Defeated "Sword Saint Isshin Ashina"
Defeated the "Demon of Hatred"
Used Lapis Lazuli to upgrade any tool to its limit
Defeated the "Divine Dragon" and obtained the "Divine Dragon's Tears"
Acquired all Ninjutsu Techniques
Used the Mortal Blade to sever the Guardian Ape's undying
Fully upgraded the "Healing Gourd"
Defeated the "Great Colored Carp".
Defeated the "Corrupted Monk"
Traveled to all areas of the game
Defeated "Great Shinobi - Owl"
Acquired all Prosthetic Tools
Defeated the "Guardian Ape"
Defeated the "Great Serpent".
Caught the Folding Screen Monkeys
Defeated "Genichiro Ashina"
Grasped the inner mysteries of any combat style
Defeated "Gyoubu Masataka Oniwa"
Encountered the Memorial Mob
Defeated "Lady Butterfly"
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
Acquired all skills
Attained the "Immortal Severance" ending
Attained the "Shura" ending
Defeated "Isshin Ashina"