At first place I was wondering why I couldn't see your deprecation warning building RSL against your branch: It picked the tl_expected header from rcpputils. I did not know before that this was packaged there. Should we maybe simply remove it here and let ROS PMC handle this in rcpputils?

Changing the includes from
#include <tl_expected/expected.hpp>
to
#include "tl_expected/expected.hpp"
used that of my local workspace and had the desired deprecation warning.

Thanks for the insight.
Looking into this, it seems rcpputils/tl_expected/expected.hpp was added to internalize the dependency for point_cloud_transport as a core package (ros2/ros2#1516, https://github.com/ros-perception/point_cloud_transport/pull/48/changes). It's not referenced from any of rcpputils' own public headers — it's only used via #include <rcpputils/tl_expected/expected.hpp> in point_cloud_transport's public headers. So the number of external packages depending on it directly should be small, though including point_cloud_transport headers would still trigger the same header guard conflict.

On the other hand, cpp_polyfills' tl_expected has direct downstream dependents, so it can't simply be removed.

Regarding the deprecation warning not appearing — I'll investigate later.

Sorry, I tried now to just include rcpputils, and it did not work without adapting the includes to <rcpputils/..> as you pointed out: PickNikRobotics/RSL#157

The USE_VENDORED_TL_EXPECTED option was originally added for users who depend on a newer version of tl_expected (PickNikRobotics/generate_parameter_library#311 (comment)). However, since tl_expected is no longer receiving new releases and it should be backward compatible, I think it's safe to remove this option. I'll update the PR accordingly.

Sorry, the deprecation warning logic was reversed in the previous push. Fixed now.