At least some of that will be in the SRTM heights - the reflection tends to come from upper edges of forests and BUA.. but that does mean that ground levels are poorly represented. However the difference is usually only 5-15m depending on what type of cover is present (large city centre buildings excepted).

More significant to LOS/noise might be the linear features that will not really show up at all - powerlines, walls, hedges, fences etc, as these are both more prevalent (in the UK I'd estimate around 50% of most 'long distance' fields of view will terminate in some form of linear feature (or linear feature around a wood/BUA) rather than the ground surface - and a lot of ridgelines are bisected by roads/paths/field boundaries).

OTOH, most long range antenna are mounted significantly higher than ground level - so it might be sufficient to just reduce the antenna 'height above ground' by a position dependant factor to account for local man-made clutter and ground cover. This might mean no reduction for flat hard steppe, and a reduction of 'most' of the antenna height for western European wooded/built up terrain. Of course YMMV - this is just off-the-cuff and no more than supposition.