Hello Stefan,
I have no knowledge of this matter, but from what I understand from
sse2-support package page (1), by forbidding its installation you will get rid
of your error message without being rid of the error: that is précisely its
purpose to warn you, by failing to install, that a package that has a
dependancy on the sse2-support package needs your processor to support SSE2.
I suppose you could then, by forbidding sse2-support package install, authorize
the install of a package that depends on it (apt redpends sse2-support should
give you a list of candidates) and the executable could crash, run slowly, or
worse: fail silently by giving you results that seem right but are wrong.
(1) https://packages.debian.org/buster/sse2-support
