Linux 4.19 has been packaged for Debian GNU/Linux 9 LTS as linux-4.19 to provide a supported upgrade path for systems that currently use kernel packages from the stretch-backports suite.

DLA 2324-1: linux-latest-4.19 new package

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Debian LTS Advisory DLA-2324-1 debian-lts@lists.debian.org
  https://www.debian.org/lts/security/
August 12, 2020   https://wiki.debian.org/LTS
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Package : linux-latest-4.19
Version : 105+deb10u5~deb9u1

Linux 4.19 has been packaged for Debian 9 as linux-4.19. This
provides a supported upgrade path for systems that currently use
kernel packages from the "stretch-backports" suite.

However, "apt full-upgrade" will *not* automatically install the
updated kernel packages. You should explicitly install one of the
following metapackages first, as appropriate for your system:

linux-image-4.19-686
linux-image-4.19-686-pae
linux-image-4.19-amd64
linux-image-4.19-arm64
linux-image-4.19-armmp
linux-image-4.19-armmp-lpae
linux-image-4.19-cloud-amd64
linux-image-4.19-marvell
linux-image-4.19-rpi
linux-image-4.19-rt-686-pae
linux-image-4.19-rt-amd64
linux-image-4.19-rt-arm64
linux-image-4.19-rt-armmp

For example, if the command "uname -r" currently shows
"4.19.0-0.bpo.9-amd64", you should install linux-image-4.19-amd64.

There is no need to upgrade systems using Linux 4.9, as that kernel
version will also continue to be supported in the LTS period.

Further information about Debian LTS security advisories, how to apply
these updates to your system and frequently asked questions can be
found at:   https://wiki.debian.org/LTS