Technically speaking Aruba 3810M Switch series provides a Slot on the back for additional high speed ports. Not like having a full modular chassis like the one provided by Aruba 5400R zl2 Switch series but enough in some scenarios. To me modularity copes with Management Module(s), not only with the fact that you have various interface modules...from this point of view Aruba 3810M is not entirely modular even if is lightly (interface) modular.
Really modular are Aruba 5400R zl2 and its counterpart Aruba CX 6400.
Worth to note that, speaking about Aruba 3810M and Aruba CX 6400 v1, for both there is an End of Sales date and an End of Support date, respectively:
Aruba 3810M --------> End of Sale on Jun 30, 2023 and End of Support on Jun 30, 2028
Aruba CX 6400 v1 ---> End of Sale on Feb 28, 2023 and End of Support on Feb 28, 2028 (the Aruba CX 6400 v2 is already available).