In experimental there is now
luatex 1.11.0 with harfbuzz 2.5.1
It follows from the development of harftex by K. Hosny.
Please note that the code is
1) in experimental
2) marked as work in progress
so it can stay as is, it can be changed , it can be removed completely.
In experimental and in texlive there are now luatex & luahbtex and jit variants.
Briefly, after some discussions emerged the point that luatex with hb statically linked
very likely doesn't work in workflows that already use luatex with a dynamic binding with a dll/so version of harfbuzz,
even considering the API stability of harfbuzz itself and the fact that now the hb symbols are hidden.
In this way we preserve the workflows that use luatex, because there is no harfbuzz code in luatex.
It's better to think luahbtex as "luatex + a binding to harfbuzz."
and not as a variant of luatex adapted to harfbuzz.
So any change in luatex will be in luahbtex, because there is a single source.
On the other side, the only constrain on the hb binding is that it cannot modify the luatex source,
so in principle it can grow to cover all the harfbuzz API --- there are of course others constrains to consider
as maintenance and compatibility with formats, just to say the most obvious.