The desired embedding is indeed correct for $\theta = 1-1/p$. This is a classical result in interpolation theory and the theory of evolution equations. See for example the book of Amann [2], Theorem III.4.10.2.$^1$

In fact, the real interpolation space $$X:= \bigl(X_0,X_1\bigr)_{1-1/p,p}$$ can be *defined* (equivalently to several other methods) as the space of point evaluations of zero of functions in the intersection space. This is called the (Lions-Peetre) **trace method** which you will find in nearly any book on interpolation theory. (For instance in Bergh/Löfström [1], Corolllary 3.12.3) From there, one uses e.g. continuity of shift semigroups to obtain the desired embedding.

If, as quite often when dealing with the spaces in the question, $X_1 \hookrightarrow X_0$, then the embedding is true for all $0 \leq \theta \leq 1-1/p$ by factoring through $\theta=1-1/p$ and a natural embedding of interpolation spaces. ("Take more of the bigger space $X_0$".) However, if you are content with $\theta < 1-1/p$, then you can in fact do better and even have Hölder-continuity in time of order $\alpha = 1-1/p-\theta$.

Since I also checked the previous version of your question, let me mention that results as asked there are indeed also known. A common name is *mixed derivative theorem*. (Which is unfortunate, since web searches will not be very effective.) But naturally they become much more involved considering noninteger differentiability scales. Right now I know that such results (and more or less rigorous proofs) are considered e.g. in another work of Amann [3] and by Denk and Kaip; maybe rather see the accessible version for $X_0 = L^p$ in [4], Proposition 4.3. But there are surely more.

[1] *Bergh, Jöran; Löfström, Jörgen*, Interpolation spaces. An introduction, Grundlehren der mathematischen Wissenschaften. 223. Berlin-Heidelberg-New York: Springer-Verlag.

[2] *Amann, Herbert*, Linear and quasilinear parabolic problems. Vol. 1: Abstract linear theory, Monographs in Mathematics. 89. Basel: Birkhäuser.

[3] *Amann, Herbert*, Compact embeddings of vector-valued Sobolev and Besov spaces, Glas. Mat., III. Ser. 35, No. 1, 161-177 (2000).

[4] *Tolksdorf, Patrick*, **On the $\mathrm {L}^p$-theory of the Navier-Stokes equations on three-dimensional bounded Lipschitz domains**, Math. Ann. 371, No. 1-2, 445-460 (2018).

$^1$ I am not quite sure whether this qualifies as an accessible source, since Amann's works are notoriously hard to read, but I hope this particular result should be accessible enough..