Combat. World design and lore is nowhere close but combat is similar. Parry/guard mechanics are identical, combo system, stagger gauge are the same. In Tsushima, there's a stamina limit in place where after several swings your speed and damage drop off so you have to use patience and wait for an opening and do what you can rather than Sekiro's system where a nonstop onslaught without a retreat can pay off for skilled players. That's just not possible in every fight with Tsushima, though as you get better you can learn new stances which are better at creating openings on different enemy types.

The worlds aren't similar except for theme. Map structure is closer to Creed but the traversal system in Sekiro was smoother on foot.

Tl;dr combat is extremely similar but it stops there. IMO combat is the star of the show in both games. If you liked Sekiro's combat, you'll like GoT.