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