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