TLEs and SGP4 may help.
If we calculate the mean radius vector (or the semi-major axis, but not the osculating semi-major axis) for the TLE epoch, we get the following graph:

The reboosts are clearly shown, but there is no deboost (the spikes you see immediately after a reboost are TLE artefacts).
EDIT
This first part of the edit is to clarify about the @uhoh’s misleading message where he say that in this graph:

there is a sudden radius vector drop, while now I’m saying that there are no sudden drops.
It’s absolutely clear that the sudden drop shown in the second graph is caused by a big increase of the air density and not by a “propulsive orbit-lowering executed intentionally” (aka deboost).
Since he asked: "How often has a propulsive orbit-lowering been executed intentionally?", I’m just saying that for the graphed period there are no “propulsive orbit-lowering executed intentionally”.
All that said, a good resource could be the page https://spaceflight.nasa.gov/realdata/sightings/SSapplications/Post/JavaSSOP/orbit/ISS/SVPOST.html
where there are the lines:
IMPULSIVE TIG (GMT) M50 DVx(FPS) LVLH DVx(FPS) DVmag(FPS)
IMPULSIVE TIG (MET) M50 DVy(FPS) LVLH DVy(FPS) Invar Sph HA
DT M50 DVz(FPS) LVLH DVz(FPS) Invar Sph HP
------------------------------------------------------------------------
361/03:07:48.737 -1.0 2.1 2.1
N/A -0.5 -0.2 221.5
000/00:05:37.474 1.8 -0.1 215.8
For a deboost, we probably should expect a negative LVLH DVx component (but I’m not totally sure).
Afaik, only the current version of that page seems to be available, but I’m saving a local copy of that page since the day 120 of the year 2017; here’s an example of the list of the maneuvers:
IMPULSIVE TIG (GMT) DT DVx DVy DVz DVmag
137/22:15:09.861 000/00:00:19.723 1.0 0.0 0.0 -> 1.00
…
361/03:07:48.737 000/00:05:37.474 2.1 -0.2 -0.1 -> 2.11
the full list can be downloaded here.
The method TLE+SGP4 seems better to me because also the very old TLEs are available. Moreover, if we plot the orbital speed:

we can obtain a good estimation of the deltaV also for values as small as 50 mm/s or better.