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