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