There is a theory in the real world that an infinite number of universes exist which ultimately means there are an infinite number of versions of every single person that has ever lived.
The theory even goes as far as to say that, since there are an endless number, there are some versions of people out there that are identical in every single way. Multiple versions of the same person do the same things down to every eye movement, every twitch, and every thought.
If that theory were to be applied to the MCU, then it is a guarantee that there are an infinite number of Kang the Conqueror Variants that act as a He Who Remains, just as there are an infinite number of Kang Variants that are the same as Victor Timely and every other Variant of the character.
Since the Multiverse is, in fact, infinite in the MCU, then there has to be other versions of He Who Remains out there somewhere. Likely, none of them will ever be seen in the MCU, but it is worth noting that they are likely out there.
So, while the version of He Who Remains in Loki does seem to be truly dead, it is fun to think about there being an infinite number of He Who Remains out there at different points in the universe, and even more fun to think about how some versions of Loki made different decisions than the MCU Loki did and are likely beside their respective versions of He Who Remains.