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