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"
Grasped the inner mysteries of any combat style
Defeated "Gyoubu Masataka Oniwa"
Acquired the Shinobi Prosthetic
Received the "Kusabimaru" from Kuro
All achievements have been unlocked.
Defeated all bosses
Traveled to all areas of the game
Upgraded all Prosthetic Tools to their limit
Acquired all skills
Acquired all Prosthetic Tools
Acquired all Ninjutsu Techniques
Upgraded Vitality and Posture to their limit
Fully upgraded the "Healing Gourd"
Attained the "Purification" ending
Attained the "Return" ending
Attained the "Shura" ending
Used Lapis Lazuli to upgrade any tool to its limit
Encountered the Memorial Mob
Returned from the dead using "Resurrection" for the first time
Defeated "Lady Butterfly"
Defeated "Genichiro Ashina"
Defeated the "Guardian Ape"
Used the Mortal Blade to sever the Guardian Ape's undying
Caught the Folding Screen Monkeys
Defeated "Great Shinobi - Owl"
Defeated "Great Shinobi - Owl" at the Hirata Estate
Defeated the "Corrupted Monk"
Defeated the "Divine Dragon" and obtained the "Divine Dragon's Tears"
Defeated "Isshin Ashina"
Defeated the "Great Serpent".
Defeated the "Great Colored Carp".