Install this version:
emerge -a =dev-util/msbuild-meta-15.9
# Copyright 1999-2021 Gentoo Authors
# Distributed under the terms of the GNU General Public License v2
EAPI="7"
KEYWORDS="amd64 arm64"
SLOT="0"
# it's impossible to use
# inherit msbuild-framework
# ?DEPEND=" ... dev-dotnet/gentoo-net-sdk[$(msbuild_expand msbuild15-9)]"
# will say
# (masked by: invalid: DEPEND: Invalid atom (dev-dotnet/gentoo-net-sdk[), token 3,
# invalid: RDEPEND: Invalid atom (dev-dotnet/gentoo-net-sdk[), token 3)
#
# that's why the full name is used for USE-flag "msbuild_targets_msbuild15-9"
# dev-dotnet/gentoo-net-sdk[msbuild_targets_msbuild15-9]
CDEPEND="
>=dev-util/msbuild-15.9
app-eselect/eselect-msbuild
dev-dotnet/gentoo-net-sdk[msbuild_targets_msbuild15-9]
"
RDEPEND="${CDEPEND}
"
DEPEND="${CDEPEND}
"
# >=dev-dotnet/msbuildtasks-1.5.0.240-r1
>=dev-util/msbuild-15.9 app-eselect/eselect-msbuild dev-dotnet/gentoo-net-sdk[msbuild_targets_msbuild15-9]
>=dev-util/msbuild-15.9 app-eselect/eselect-msbuild dev-dotnet/gentoo-net-sdk[msbuild_targets_msbuild15-9]